22 



HUMAN EMBEYOLOGY. 



Trophoblast 



Extra-, 

 embryonic 

 entoderm 

 Primary mesoderm 



Plasmodial trophoblast 



Cellular trophoblast 



Amniotic ectoderm 



Embryonic % 

 ecto-mesoderm T 



inner mass become converted into hollow vesicles by the appearance of cavities in 

 their interiors (Fig. 29). 



When the above-mentioned changes have occurred the zygote consists of three 

 spheres, one large and two small. The large sphere is bounded by the tropho- 

 blast ; it contains the two small spheres and the 

 jelly-like mass of primary mesoderm derived from 

 the primary mesoderm segment of the inner mass 

 (Fig. 29). 



The two small vesicles lie ex-centrically in the 

 interior of the larger vesicle. The larger and more 

 external of the two is the ecto-mesodermal vesicle. 

 It is separated from the trophoblast, peripherally, 

 and the entodermal vesicle, centrally, by the sur- 

 rounding primary mesoderm. 



The early appearance of the mesoderm in the 

 FI. 28.-FURTHER DIFFERENTIATION Z 7g ote and its insinuation at so early a period be- 

 OF ZYGOTE (Hypothetical). tween the ectoderm and the entoderm are peculiar- 



ities limited to the human subject. In most mammals 

 the mesoderm does not appear until the 

 embryonic area and its primitive streak are 

 defined. 



The Embryonic Area. The area where 

 the two inner vesicles he in apposition with 

 each other is the region of the zygote from 

 which the embryo will be formed ; it is 

 called, therefore, the embryonic area, and at 

 the time of its definition it consists of three 

 layers, ectoderm, primary mesoderm, and 

 entoderm. It is uncertain whether the 

 mesoderm which is present in the area at 

 this period takes part in the formation of 

 the embryo or is replaced at a later period 

 by mesoderm 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 

 Ccelom. The extra - embryonic 

 coelom is a space which appears 

 as two clefts, one on each side of 

 the embryonic area, in the primary 

 mesoderm (Fig. 30). The clefts 

 fuse together round the periphery 

 of the embryonic area, and the 

 single space so formed expands 

 rapidly until the mesoderm which 

 originally filled the greater part 

 of the larger vesicle becomes con- 

 verted into a thin layer which 

 lines the inner surface of the 

 trophoblast and covers the outer 

 surfaces of the epithelial walls of 

 the extra-embryonic parts of the 

 two inner vesicles (Fig. 32). 

 The extra -embryonic coelom does not extend into the embryonic area, and 

 it never completely separates the ecto-mesodermal vesicle from the inner surface 

 of the trophoblast ; on the contrary, the primary mesoderm on the outer surface 

 of the ecto-mesodermal vesicle retains its continuity with the mesoderm on the 

 inner surface of the trophoblast until the termination of intrauterine life, and 



Extra- 

 embryonic 

 entoderm 



FIG. 29. SCHEMA OF DIFFERENTIATION OF 

 ZYGOTE (Bryce's Ovum). 



Plasmodial trophoblast 

 Cellular trophoblast \ 



Amniotic ectoderm -V-* 



Amnion cavity^ 

 Embryonic 

 ecto-niesoderm^ 



Extra-embryonic <^1 

 creloin j 



Primary mesoderm;- 



FIG. 30. SCHEMA OF DIFFERENTIATION OF ZYGOTE 

 (Peter's Ovum). 



