28 



CLEAVAGE AND THE GERM LAYERS 



phectoderm of the 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 cleft-like blastocasle of the bird's ovum. The 

 mammalian ovum, although almost devoid of yolk, thus develops much 



Blastula cavity 



D 



Yolk cavity 



PIG. 18. Diagrams showing the blastulae: A, of Amphioxus; B, of frog; C, of chick; D, blasto- 



dermic vesicle of mammal. 



like the yolk-laden ova of reptiles and birds. This similarity has an 

 evolutionary significance. Its cleavage, however, is complete and the 

 early stages in its development are abbreviated. 



In Primates, but one cleavage stage 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 uterine tube of the monkey and shows that, in Primates and 

 probably in man, cleavage as in other mammals take place normally in the oviducts. 



THE FORMATION OF ECTODERM AND ENTODERM (GASTRULATION) ^ 

 The blastula and early blastodermic vesicle show no differentiation 

 into layers. Such differentiation takes place later in all vertebrate em- 

 bryos, giving rise first to the ectoderm and entoderm, and finally to the 

 mesoderm. From these three primary germ layers all tissues and organs 

 of the body are derived. 



The processes of gastrulation, by which ectoderm and entoderm arise, 

 and of mesoderm formation will be treated separately. 



