10 



MORPHOGENESIS 



cavity; the deeper (D) is the cavity of the yolk-sac; while between them is a 

 plate of cells forming the embryonic disc. The embryonic disc (figs. 9 and 10) 

 contains three layers of cells, — the fundamental germ layers, — ectoderm (Ec), 

 endoderm (En), and mesoderm. 



The germ layers of the embryonic disc are of prime importance in the development of the 

 body. From the ectoderm, which lies next to the amniotic cavity and represents the upper 

 (later outer) germ layer, are derived the epidermis and the entire nervous system. From the 

 endoderm, which lies next to the yolk-sac, and represents the lower (later inner) germ layer, is 

 derived the epitheUal lining of the digestive mucosa and its derivatives. From the mesoderm, or 

 middle germ layer, is differentiated the remainder of the body, including the skeletal and sup- 

 porting tissues, vascular system, muscle and most of the urogenital organs. 



The germ layers also extend beyond the embryonic disc, as shown in figs. 9 and 10. The 

 yolk-sac is made up of a Uning of endoderm and an outer layer of mesoderm. The amnion, which 



Fig. 10.- 



-DiAGRAM OF Section of a Mammalian Ovum showing the Embryonic Disc, 

 Amniotic Cavity and the Germ Layers. 



Endoderm 



Yolk-sac 



later becomes separated from the chorion, is composed of mesoderm lined by endoderm. The 

 outer cell layers form the chorion, which likewise shows two layers, the outermost of which 

 (trophoblast) is ectoderm, the inner,|mesoderm. In fig. 10 the chorion is beginning to send out 

 root-like projections (viUi) which invade the uterine mucosa. 



It is thus noteworthy that of the cells derived from the ovum relatively only a few — those of 

 the embryonic disc — enter directly into the formation of the body. The yolk-sac, a rudimentary 

 organ of phylogenetic significance, is later chiefly absorbed, although the proximal portion may 

 enter slightly into the formation of the intestinal wall. The amnion is a protective membrane, 

 while the chorion forms the foetal part of the placenta. 



Development of the embryonic disc. — When first formed, the surface of the 

 embryonic disc shows no trace of differentiation. A slightly later but still 

 comparatively early stage in its development is shown in fig. 11. It is here 



Fig. 11. — Model Showing the Embryonic Disc from an Embryo 1.17 mm. In Length. 

 Viewed from above and laterally, the roof of the amniotic cavity having been removed, n, 

 primitive pit (neurenteric canal), -pg, primitive groove, mg, neural groove, h, body-stalk. 

 (McMurrich. from Frassi.) 



viewed from above, the amnion having been removed. The disc is an elliptical 

 plate, whoso long axis represents the mid-line of the embryo. Near the center 

 is a small rounded depression, the 'primitive pit. Extending backward (toward the 

 tail end of the embryo) from this is a dark line, the primitive streak, corresponding 

 to a groove, the primitive groove. Extending forward from the primitive pit is an 

 indistinct wide shallow groove, the neural groove. 



At an earlier stage, the primitive streak extends further forward, possibly to the anterior end 

 of the embryonic disc (Spee). The primitive streak and groove probably correspond to the 



