REPRODUCTION 



77 



and forward from the primitive streak and always in the space between 

 ectoderm and endoderm. This layer, like the early mesoderm of amphib- 

 ians, is unsegmented and devoid of cavity. 



In the sauropsidan embryo, then, as in the amphibian, rapid growth 

 and cell proliferation within the blastoporal rim is the primary source of 

 mesoderm. 



Early Development in Placental Mammals 



The early development of placental mammals exhibits features peculiar 

 to the group and more or less difficult of comparison with anything in the 

 development of lower vertebrates. The minute egg (Figs. 28 and S3) 

 contains a bare minimum of yolk. Cleavage is total, more or less unequal 

 and often very irregular in respect of planes and sizes of cells (Fig. 51.4). 



CV- 



A. B. 



Fig. 51. — Early stages in development of a rabbit. A, morula stage, 47 hours after 

 coitus; B, early blastodermic vesicle, 80 hours; C, blastodermic vesicle at 83 hours. 

 The investing layers of the embryo are not shown. CV, cavity of blastodermic vesicle; 

 7, inner cell-mass; T, trophoblast. Magnified about 285 diameters. (After Assheton.) 



The cells resulting from cleavage remain in a sohd cluster, the "morula," 

 until as many as sixty or seventy cells are present. Then, as the number 

 increases further, a cavity appears within the morula (Fig. 51^, C). 

 Most of the cells remain in a solid group at one side of the cavity whose 

 wall elsewhere is only one cell thick. At this stage the embryo looks like a 

 blastula, but further development proves that the stage is not the equiva- 

 lent of a blastula of a lower vertebrate. The term, blastodermic vesicle, 

 is applied to this stage of the mammalian embryo. The definitive embryo 

 is developed entirely from the thick cell-mass of the vesicle. The thin 

 region (trophoblast. Fig. 51, T) of the wall of the vesicle becomes concerned 

 with the early attachment of the embryo to the wall of the uterus. 



The fluid-filled cavity of the blastodermic vesicle rapidly enlarges 

 and meanwhile the thick cell-mass sphts off a thin layer adjoining the 

 cavity (Fig. 52). This inner sheet of the thick mass then extends over the 

 inner surface of the thin wall of the vesicle and ordinarily completely 

 fines it. The vesicle as a whole thereby becomes two-layered throughout, 

 a condition which characterizes a gastrula stage. The further history 

 of the two layers identifies them as embryonic ectoderm and endoderm. 



