THE FORMATION OF THE ECTODERM AND ENTODERM 37 



mammal represents a precocious development of cells which, in the bird, later 

 envelop the yolk. The cavity of the vesicle is to be compared, not with the 

 blastula cavity of Amphioxus and the frog but with the yolk mass plus the rudi- 

 mentary blastocoel of the bird's ovum. The mammalian ovum, although almost 

 devoid of yolk, thus develops mtich like the yolk-laden ova of reptiles and birds. Its 

 segmentation, however, is complete and the early stages in its development are 

 abbreviated. 



A b 



oj Blastula cavity 



D 



Yolk cavity 



Fig. 17. — Diagrams showing the blastulae: A, of Amphioxus; B, of frog, and C of chick; D, blastodermic 



vesicle of mammal. 



In Primates, but one stage in segmentation has been observed. This, a four-celled ovum 

 of Macacus nemestrinus figured by Selenka, shows the cells nearly equal and oval in form. 

 This ovum was found in the oviduct of the monkey and shows that, in Primates and probably 

 in man, segmentation as in other mammals takes place normally in the oviducts. 



THE FORMATION OF THE ECTODERM AND ENTODERM 

 The blastula and early blastodermic vesicle show no differentiation into 

 layers. Such differentiation takes place later in all vertebrate embryos and the 

 three primary germ layers, ectoderm, entoderm and mesoderm, are formed. From 

 these three layers all of the body tissues and organs are derived. 



Gastrulation. — In the case of Amphioxus and amphibia the entoderm is 



