52 INTRODUCTION TO EVOLUTION 



ity is called the amnion, the lower one the yolk sac, and the double layer 

 separating them is referred to as the embryonic disc. The embryo itself 

 forms from this embryonic disc, the two layers of which are the ectoderm 

 and the endoderm. Hence this two-layered stage of the human embryo cor- 

 responds to the gastrula stage of typical development (Fig. 4.2), although 

 it differs from the typical form in appearance and in method of formation. 



embryonic 



ectoderm 

 endoderm 



piV — ^olk sac 



trophoblast 



chorionic villi 



extra embryonic 

 coelom 



FIG. 4.3. Human embryonic development during the first week following fertilization, 

 (C, after Patten, Human Embryology, The Blakiston Company, 1946.) 



There is interest in the fact that gastrulation and mesoderm formation in 

 the human embryo, as in the embryos of other mammals, are more like 

 these processes in large-yolked eggs (reptiles, birds) than they are like the 

 processes in small-yolked ones (e.g., Fig. 4.2). This is true despite the 

 absence of yolk. Why? (See discussion of the yolk sac, pp. 168-169.) 

 During the first few days the embryo grows with great rapidity (Fig. 

 4,4). In the figure most of the trophoblast has been removed, only the 



