THE DEVELOPMENT OF THE INDIVIDUAL 



537 



Early Embryogenesis of the Mammals 



In the mammals special problems in embryonic development arise, 

 and there are certain differences in the early stages. The mammal egg 

 is small, and there is a small amount of yolk. The cleavage of the 

 zygote is equal and total in the early stages, and a mass of cells of about 

 equal size is formed. As divisions continue this mass forms an outer 



Photos by P. W. Gr 



Contribution to Embryology 



Fig. 33.9. Early embryology of a mammal. These photomicrographs of living rabbit 

 embryos are very much like similar stages in human development. The upper row 

 shows a morula 55 hours after coitus ; morula 72 hours after coitus ; beginning of 

 blastula formation 77 hours after coitus. The lower row shows a fully formed blastula 

 with blastoderm near the top and the blastodermic vesicle filling the center of the 

 ball at 90 hours. Also, a more advanced blastula at 92 hours. 



layer, the trophoblast layer, and an inner cell mass. This entire body 

 is known as the blastodermic vesicle. The embryo and the extraem- 

 bryonic membranes develop from the inner cell mass. The trophoblast 

 layer contributes to the formation of the chorion in combination with 

 certain mesodermal cells which come to underlie it. From this point on 

 the mammals show no great peculiarities in the development of their 

 organs in comparison with the other chordates. 



