CLEAVAGE 



27 



Among Ungulates (hoofed animals) the vesicle is greatly elongated and attains a 

 length of several centimeters, as in the pig. 



If we compare the mammalian blastodermic vesicle with the blastula stages 

 of Amphioxus, the frog, and the bird, it will be seen that it is to be homologized 

 with the bird's blastula, not with that of Amphioxus (Fig. 18). In each case 

 there is an inner cell mass of the germinal disc. The trophectoderm 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 



D 



Yolk cavity 



Fig. 18. — Diagrams showing the blastulae: A, of Amphioxus; B, of frog; C, of chick; D, blastodermic 



vesicle of mammal. 



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

 mentary blastoccele of the bird's ovum. The mammalian ovum, although almos t 

 c^evoid ^Jifjyolk^us develo^ much like the yolk-laden ova _o£re^les 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 prob- 

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



