556 



ZOOLOGY 



SECT. 



The outer layer is the trophoUast. Immediately beneath it, 

 throughout its extent, a thin layer of flattened cells appears- 

 the peripheral endoderm and this becomes continuous with a 

 similar layer formed on the inner surface of the embryonic cell-mass 



the embryonic endoderm. The rest of the embryonic cell-mass gives 



rise to the embryonic ectoderm. The part of the trophoblast lying 

 over this embryonic ectoderm thins out and disappears, or fuses 

 with it, and the embryonic ectoderm becomes continuous at its 

 margin with the remainder of the trophoblast, which forms the 

 outer layer of the rest of the wall of the blastodermic vesicle. 



Fir 1147 Embryonic area of a seven days' embryo Babbit, ac/, embryonic area ; o, place of 

 "future vascular area ; pr. primitive streak ; rf, medullary groove. (From Balfour after 

 Kolliker.) 



On the embryonic area a primitive streak (Fig. 1147, pr.) 

 and primitive groove are developed very much as in the Bird. A 

 medullary groove (rf) and canal are formed in front of the 

 primitive groove, and a row of protovertebras (Fig. 1148) make 

 their appearance on each side of the former. The embryo be- 

 comes folded off from the blastoderm as in the Bird, and at 

 length the body of the young Mammal becomes constricted off 

 from the yolk-sac or umbilical vesicle, so that, ultimately, the two 

 come to be connected only by a narrow yolk-stalk (Figs. 1149 and 

 1150): the yolk-sac is a thin-walled sac containing a coagulable 

 fluid in place of yolk. A vascular area early becomes established 

 around the embryo on the wall of the yolk-sac. 



