SEGMENTATION OF THE FERTILIZED OVUM 



47 







outer, termed the syncytium or syncytiotrophoblast, so named because it consists of 

 a layer of protoplasm studded with nuclei, but showing no evidence of subdivision 

 into cells; and an inner layer, the cytotrophoblast or layer of Langhans, in which 

 the cell outlines are defined. As already stated, the cells of the trophoblast do not 

 contribute to the formation of the embryo proper; they form the ectoderm of the 

 chorion and play an important part in the development of the placenta. On the 

 deep surface of the inner cell-mass a layer of flattened cells, the entoderm, is differ- 

 entiated and quickly assumes the form of a small sac, the yolk-sac. Spaces appear 

 between the remaining cells of the mass (Fig. 11), and by the enlargement and 

 coalescence of these spaces a cavity, termed the amniotic cavity (Fig. 12), is gradually 

 developed. The floor of this cavity is formed by the embryonic disk composed 

 of a layer of prismatic cells, the embryonic ectoderm, derived from the inner cell- 

 mass and lying in apposition with the entoderm. 



The Primitive Streak; Formation of the Mesoderm. The embryonic disk 

 becomes oval and then pear-shaped, the wider end being directed forward. Near 

 the narrow, posterior end an opaque streak, the primitive 

 streak (Figs. 13 and 14), makes its appearance and extends 

 along the middle of the disk for about one-half of its 

 length; at the anterior end of the streak there is a knob- 

 like thickening termed Hensen's knot. A shallow groove, 

 the primitive groove, appears on the surface of the streak, 

 and the anterior end of this groove communicates by 

 means of an aperture, the blastophore, with the yolk-sac. 

 The primitive streak is produced by a thickening of the 

 axial part of the ectoderm, the cells of which multiply, 

 grow downward, and blend with those of the subjacent 

 entoderm (Fig. 15). From the sides of the primitive streak 

 a third layer of cells, the mesoderm, extends lateralward 

 between the ectoderm and entoderm; the caudal end of 

 the primitive streak forms the cloaca! membrane. 



The extension of the mesoderm takes place throughout the w r hole of the embry- 

 onic and extra-embryonic areas of the ovum, except in certain regions. One of 

 these is seen immediately in front of the neural tube. Here the mesoderm extends 

 forward in the form of two crescentic masses, w r hich meet in the middle line so as 

 to enclose behind them an area which is devoid of mesoderm. Over this area the 

 ectoderm and entoderm come into direct contact with each other and constitute 

 a thin membrane, the buccopharyngeal membrane, which forms a septum between 

 the primitive mouth and pharynx. In front of the buccopharyngeal area, where 

 the lateral crescents of mesoderm fuse in the middle line, the pericardium is 

 afterward developed, and this region is therefore designated the pericardial area. A 

 second region where the mesoderm is absent, at least for a time, is that imme- 

 diately in front of the pericardial area. This is termed the proamniotic area, and 

 is the region where the proamnion is developed; in man, however, a proamnion is 

 apparently never formed. A third region is at the hind end of the embryo where 

 the ectoderm and entoderm come into apposition and form the cloacal membrane. 



The blastoderm now consists of three layers, named from without inward: 

 ectoderm, mesoderm, and entoderm; each has distinctive characteristics and gives 

 rise to certain tissues of the body. 1 



Ectoderm. The ectoderm consists of columnar cells, which are, however, somewhat 

 flattened or cubical toward the margin of the embryonic disk. It forms the whole 

 of the nervous system, the epidermis of the skin, the lining cells of the sebaceous, 



1 The mode of formation of the germ layers in the human ovum has not yet been observed; in the youngest known 

 human ovum (viz., that described by Bryce and Teacher), all three layers are already present and the mesoderm ia 

 split into its two layers. The extra-embryonic celom is of considerable size, and scattered mesodermal strands are 

 seen stretching between the mesoderm of the yolk-sac and that of the chorion. 



FIG. 13. Surface view of 

 embryo of a rabbit. (After 

 Kolliker.) arg. Embryonic 

 disk. pr. Primitive streak. 



