Reproduction 263 



Fig. 217. 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. Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The 

 Rlakiston Company.) 



together without sharp demarcation (Fig. 212). Following gastrulation, 

 the blastodermal layers continue to spread rapidly over the surface of 

 the yolk. In so doing, the growth posteriorly causes the somewhat 

 thickened region of the blastoporal rim to become drawn out into a 

 long streak, the primitive streak, lying in the median line of the 

 blastoderm (Fig. 216). Along the whole extent of this modified blasto- 

 poral region, the ectoderm and endoderm merge without sharp demar- 

 cation, just as they did in the earlier blastoporal walls (Fig. 217). 



This primitive streak is the primary seat of mesoderm formation. 

 Rapid proliferation of cells within the substance of the thickened streak 

 gives rise to masses of cells which move out into the space between 

 ectoderm and endoderm (Fig. 217, MES). These masses of cells increase 

 by continued contribution from the streak and by growth within them- 

 selves, and soon become arranged in a layer which rapidly grows later- 

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

 between ectoderm and endoderm. This layer, like the early mesoderm 

 of amphibians, is at first 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 

 (Fig. 202) contains a bare minimum of yolk. Cleavage is total, more or 

 less unequal, and often very irregular in respect to planes and sizes of 

 cells (Fig. 218 A). The cells resulting from cleavage remain in a solid 

 cluster, the morula, until as many as 60 or 70 cells are present. Then, 

 as the number increases further, a cavity appears within the morula 

 (Fig. 218B 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 



