REPRODUCTION 



55 



derm and endoderm (Fig. 46, MES). These masses of cells increase by 

 continued contribution from the streak and by growth within themselves 

 and soon become arranged in a layer which rapidly grows laterally 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 at first unsegmented and devoid of cavity. 



Y- -^.'r^r^,**,' 



Fig. 46. — Section transverse to the primitive streak of a chick embryo of about 15 

 hours incubation. The section is taken near the middle of the length of the streak. EC, 

 ectoderm; EN, endoderm; MES, mesoderm; PG, primitive groove of primitive streak; 

 y, yolk at inner margin of area opaca. X 100. (After Duval, "Atlas d'Embryologie.") 



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 any- 

 thing in the development of lower vertebrates. The minute egg (Fig. 29) 



CV_- 



A. 



Fig. 47. — 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 einbryo are not shown. CV, cavity of blastodermic vesicle; 

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



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. 47.-I). 

 The cells resulting from cleavage remain in a solid 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. ^jB-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 



