540 VERTEBRATE LIFE AND ORGANIZATION 



Embryonic d.iSC 



AmnioTT.- 

 (cut edge) 



r Primitive. 

 strecLK 



Yolk sa-cJ-J 



-Body staJK 



A 



Ectoderra- 



Alla.ntois 



rPi^imitive 

 ^trea.H 



EiTLbryonic 

 disc 



Endod.(2rm- 



Mesoderm. 



B 



Figure 31.3. Mesoderm formation. A, A surface view of the embryonic disc of a 

 sixteen-day human embryo showing the primitive streak. B, A cross section through 

 the primitive streak. Prospective' mesoderm, which originally lies on the surface of the 

 embr)onic disc, moves in through the primitive streak and spreads out between the 

 ectoderm and endoderm in the manner shown by the arrows. (After Arey.) 



from the reptilian stage in the ancestry of mammals. The rest of the 

 cells of the inner cell mass are prospective ectoderm and mesoderm. An 

 amniotic cavity appears among the ectoderm cells at about the same 

 time that the yolk sac develops (Fig. 31.2). The double-layered plate of 

 cells lying between the yolk sac and amniotic cavity is the embryonic 

 disc. A primitive streak, similar to that of reptiles and birds, develops 

 upon the upper surface of the embryonic disc, and establishes the 

 longitudinal axis of the embryo (Fig. 31.3). Cells destined to become 

 mesoderm move inward through, and perhaps proliferate from, the 

 primitive streak. They spread out between the endoderm of the yolk 

 sac and the ectoderm that forms the surface of the embryonic disc. 

 Mesodermal involution through a primitive streak is similar to the in- 

 volution of prospective mesoderm through the blastopore of a frog, for 

 the primitive streak and the blastopore are homologous. As mesodermal 

 cells continue to spread, they form a layer beneath the trophoblastic 

 ectoderm, and this becomes a fairly typical chorion composed of ecto- 

 derm and mesoderm. Mesodermal cells also surround the endoderm of 

 the yolk sac and the ectoderm lining the amniotic cavity (Fig. 31.2). A 

 group of mesodermal cells known as the body stalk extends between 

 the embryonic disc and the chorion, and an endodermal evagination 

 grows into it from the posterior part of the yolk sac (Figs. 31.3 and 

 31.5). This evagination and the surrounding mesoderm constitute the 

 allantois. The part of the yolk sac from which this evagination arises is 



