Fertilisation is the term applied to the union to form a zygote which contains the typical nu: The meeting of the gametes and their union or middle part of the uterine tube. The details of the process are unknown in many animals it has been noted that as t the latter shows signs of excitement, and attraction, appears on its surface. At the changes of form. As the two gametes meet which surrounds the ovum and passes thro of the ovum. In some cases apparently only the ! effect an entrance, but in others the whole After the entrance occurs and before gote consists of three bounded by the trophosmall spheres and the mesoderm derived from segment of the inner mass Leslie ex-centrically in the le. The larger and more e ecto-mesodermal vesicle. -ze trophoblast, peripherally, cle, centrally, by the surderm. ce of the mesoderm in the on at so early a period bed the entoderm are peculiarsubject. In most mammals does not appear until the and its primitive streak are Embryonic Area. The area where er vesicles lie in apposition with the region of the zygote from e embryo will be formed; it is Therefore, the embryonic area, and at ts definition it consists of three ecoderm, primary mesoderm, and It is uncertain whether the which is present in the area at d takes part in the formation of To or is replaced at a later period derm derived from the cells of the ecto-mesodermal vesicle; the latter certainly forms a large part of the mesoderm of the embryo. The Extra - Embryonic on later, in the formation of the umbilical cord, which e placenta (p. 54). of the Embryonic Area.-As the embryonic area is the the ecto-mesodermal and the entodermal vesicles it is, at As growth continues the area becomes oval, and a linear streak, appears in that part of the oval which becomes the area (Fig. 31). ne the position of the mesodermal elements of the wall of the esicle is revealed, for the primitive streak is a thickened ridge of from the ecto-mesoderm and projects against the entoderm in the of the embryonic area, pushing aside the primitive mesoderm which between the adjacent parts of the walls of the ecto-mesodermal and the vesicles. The deeper cells of the ridge, those next the entoderm, are dermal elements of the primitive ecto-mesoderm, and, by proliferation, the larger part, if not the whole, of the embryonic mesoderm and also an d the notochord. The mesoderm produced from the primitive streak rmed the secondary mesoderm. Chorion Cellular trophoblast Mesoderm of amnion ediately after the formation of the primitive streak a groove, the neural appears in the anterior the embryonic area. formed by the longinal folding of a thickened te of ectoderm, the neural plate, which is the rudiment of the whole of the central and peripheral portions of the nervous system, except perhaps the olfactory nerves, and the end organs of the sensory nerves. From it also are derived the cells of the primitive sheaths of the nerve-fibres and the chromaffin cells of the Ectoderm of amnion of the cle; the supra-renal glands and other Large part chromaffin bodies. The lateral walls of the FIG. 31.-SCHEMA OF DORSAL SURFACE OF EMBRYONIC AREA OF neural groove are called the neural folds. ZYGOTE AFTER THE REMOVAL OF PART OF THE CHORION AND Their Almost from the first the anterior ends of the neural folds are united together a short distance posterior to the anterior end of the embryonic area. posterior ends, which remain separate for a time, embrace the anterior part of the primitive streak. In the meantime, however, a groove, the primitive groove, has appeared on the surface of the primitive streak. The anterior end of the primitive groove deepens, until it forms a perforation which passes, through the anterior end of the streak and the subjacent entoderm, into the cavity of the entodermal vesicle. As this perforation passes from the floor of the posterior part of the neural groove into that part of the entodermal vesicle.which afterwards becomes the primitive enteron or alimentary canal, it is called the neurenteric canal. The neurenteric canal is but a transitory passage, and it disappears in man and other mammals before the neural groove is converted into a closed neural tube. After the appearance of the primitive groove and the neurenteric canal the posterior ends of the neural folds converge, across the anterior part of the primitive streak and groove, and fuse together posterior to the neurenteric canal. The primitive streak is thus divided into two portions. (1) An anterior portion, which lies at first in the floor of the neural groove, and, later, in the floor or ventral wall of the posterior end of the spinal medulla; and (2) a posterior portion, which remains on the surface and takes part in the formation of the median portion of the posterior end of the body, forming the perineum, and the median part of the ventral wall of the body, from the perineum to the umbilicus. It is through the perineal section of the posterior part of the primitive streak that, at a later period of embryonic life, the anal and urogenital orifices of the body are formed. Anterior end of neural fold Plasmodial trophoblast Mesoderm lining of tropho Mesoderm of amnion The Formation of the Notochord and the Secondary [blast Mesoderm.-The notochord and the secondary mesoderm are formed from the primitive streak; the notochord from its anterior extremity, and the secondary mesoderm from its lateral Ectoderm of amnion Extra-embryonic colom margins and posterior end. Mesoderm covering of Plasmodial trophoblast Chorion Extra-embryonic cœlom Mesoderm of amnion Ectoderm of amnion Amnion cavity Mesoderm covering entoderm As soon as the primitive streak is established its anterior end becomes a node or centre of growth by means of which the length and, to a certain extent, the breadth of the body are increased. The portion of the body formed by the activity of the anterior end of the streak is the dorsal portion, from the back part of the roof of the nose, anteriorly, to the posterior end of the trunk. The perineum and the ventral wall of the body, from the perineum to the umbilicus, are formed from the posterior part of the primitive streak. Nevertheless, the primitive streak undergoes little or no increase in length; indeed, as growth continues, it becomes relatively shorter as contrasted with the total length of the embryonic region, for the new material, formed by its borders and its anterior extremity, is transformed into the tissues of embryo as rapidly as it is created. The Notochord. - The notochord or primitive skeletal axis is formed by the proliferation of cells from the anterior end of the primitive streak. On its first appearance it is a narrow process of cells, the head process, which projects forwards from the anterior boundary of the neurenteric canal, between the ectoderm and the entoderm. Shortly after its appearance the head process wedges its way between the entoderm cells, and from that period onwards, as the posterior parts are formed, by continued proliferation from the front end of the primitive streak, they are at once intercalated in the dorsal wall of the entodermal sac, where they remain, forming a part of the dorsal wall of the entodermal cavity (Fig. 33), for a Cavity of entodermal vesicle FIG. 34.-SCHEMA OF TRANSVERSE SECTION OF ZYGOTE ALONG LINE C IN FIG. 31. considerable time. At a later period the notochordal cells are excalated from the entoderm, and then they form a cylindrical rod of cells which occupies the median plane, lying between the floor of the ectodermal neural groove and the entodermal roof of the primitive alimentary canal, which, in the meantime, has been more or For a still less moulded off from the dorsal part of the entodermal sac (Fig. 37). longer time the caudal end of the notochord remains connected with the anterior end of the primitive streak, and its cephalic end is continuous with the entoderm of a small portion of the embryonic area, which lies immediately in front of the anterior end of the neural groove and which becomes bilaminar by the disappearance of the primary mesoderm. This region, because it afterwards forms the boundary membrane between the anterior end of the primitive entodermal canal and the primitive buccal cavity or stomatodæum, is called the bucco-pharyngeal membrane (Fig. 55, p. 42). It disappears about the third week of embryonic life, and immediately afterwards the anterior end of the notochord separates from the entoderm, but the posterior end remains continuous with the primitive streak, until the formation of the neural tube is completed. As After a time the cylindrical notochordal rod is surrounded by secondary mesoderm which becomes converted into the vertebral column of the adult. the vertebral column is formed the notochord is enlarged in the regions of the intervertebral fibro-cartilages and for a time assumes a nodulated appearance (Fig. 60). Ultimately the notochord disappears, as a distinct structure, but remnants of believed to exist as the pulpy centres of the intervertebral fibro-cartilages. The extension of the notochord into the region of the head is of interest from a It extends morphological, and possibly also from a practical point of view. through the base of the cranium from the anterior border of the foramen magnum into the posterior part of the body of the sphenoid bone. Its presence in the posterior part of the skull suggests that that region was, primitively, of vertebral nature. As the notochord passes through the occipital portion of the skull it pierces the basilar portion of the occipital region first from within outwards and then in the reverse direction. It lies, therefore, for a short distance, on the ventral surface of the rudiment of the occipital bone, in the dorsal wall of the pharynx, and it is possible that some of the tumours which form in the dorsal wall of the pharynx are due to the proliferation of remnants of its pharyngeal portion. The Differentiation of the Secondary Mesoderm. It has already been noted that a portion of the inner mass of the human zygote becomes converted directly into mesoderm which may be called, for convenience, primary mesoderm. It was stated also that the wall of the larger of the two inner vesicles of the zygote consists of ecto-mesoderm, that term being intended to convey the idea that the cells of the wall of the larger inner vesicle were the progenitors of both ectodermal and mesodermal cells. As soon as the larger of the two inner vesicles is formed two areas of its wall are defined: (1) the part in contact with the smaller inner or entodermal vesicle and (2) the remainder. As future events prove, the cells of the larger area, which is not in contact with the entodermal vesicle, simply produce ectodermal descendants which line the inner surface of a sac-like covering of the embryo termed the amnion; they are, therefore, the predecessors of the amniotic ectoderm. The cells of the larger inner vesicle, which lie adjacent to the smaller entoderm vesicle, and are merely separated from the entoderm by a thin layer of primary mesoderm, take part in the formation of the embryo; forming, with the entoderm, the embryonic area from which the embryo is evolved. These cells are the forerunners of both ectoderm and mesoderm, and as the mesoderm developed from them is differentiated after the formation of the primary mesoderm it may be termed secondary mesoderm or primitive streak mesoderm; the latter term being applied because it is differentiated in a linear region called the primitive streak (p. 23). It is the formation and fate of this primitive streak mesoderm which is now to be considered. At first the embryonic area is circular in outline, at a later period it becomes ovoid, and in the narrower or caudal portion of the ovoid area a linear thickening appears; this is the primitive streak (Figs. 31, 34). It is formed Amnion cavity by the proliferation of the ectomesodermal cells of the wall of the larger inner vesicle. The deeper cells of the streak, which displace the primary mesoderm from the median plane, and thus come into contact with the entoderm, are the rudiments of the secondary or primitive streak mesoderm (Fig. 34). The superficial cells form part of the surface ectoderm of the embryo. Notochord Mesoderm of chorion Trophoblast of chorion FIG. 35.-TRANSVERSE SECTION OF A ZYGOTE, showing early stage Amnion cavity Embryonic colom Noto- FIG. 36.-TRANSVERSE SECTION OF A ZYGOTE, showing early of mesoderm. Neural crest Neural tube, Ectoderm of amnion Mesoderm of amnion Notochord Meso. of yolk-sac Entoderm of yolk-sac Trophoblast of chorion Paraxial mesoderm Intermediate cell tract Embryonic Amnion cavity cœlom At the anterior end of the primitive streak the mesodermal elements of the streak fuse with the subjacent entoderm and through the fused mass a perforation, the neurenteric canal (p. 23), is formed (Fig. 32). The canal itself soon disappears, but the cells of its walls form a nodal growing point, and by their proliferation the length and breadth of the embryonic area are increased. The mesoderm cells proliferated from the cephalic border of the nodal point are the rudiments of the notochord, which has already been considered (p. 24). It is uncertain whether or not the mesodermal cells budded off from the nodal point blend with the cells of the primary mesoderm, but there can be little doubt that they form by far the greater part, if not the whole, of the permanent mesoderm of the embryo. Either by displacement or by union with the primary mesoderm the secondary mesoderm forms a continuous sheet of cells, in the embryonic area, on Extra-embryonic each side of the median plane. Each of the lateral sheets is thickest where it abuts against the notochord and the wall of the neural groove, and thinnest at its peripheral margin, where it is continuous with the primary mesoderm of the extra-embryonic area (Fig. 35). Alimentary canal Yolk-sac FIG. 37. TRANSVERSE SECTION OF A ZYGOTE, showing union of somatic and splanchnic layers by the intra-embryonic part of |