xni 



PHYLUM CHORDATA 



571 



enlarged anterior extremity of a primitive streak (Fig. 1228, pr.) 

 which, is developed very much in the same way as in the Bird, its 

 formation being due to the same cause as in the Bird, viz., active 

 proliferation of cells leading to the development of the beginnings 

 of the mesoderm. A dark median streak, the head-process, appears 

 in front of the primitive knot, and in some Mammals there is an 

 invagination on the surface of the latter leading to the formation 

 of a neurenteric canal and of a notochordal canal which gives rise 

 to the rudiment of the posterior part of the notochord. In the 

 region of the anterior part of the primitive streak, the primitive 

 knot .and the head-process, the mesoderm coalesces with the 

 endoderm ; but there 

 does not appear to 

 be any breaking 

 through into the un- 

 derlying space such 

 as occurs in Keptiles 

 (p. 353). A medul- 

 lary groove (rf) and 

 canal are formed in 

 front of the primitive 

 streak, and a row of 

 protovertebrae (Fig. 

 1229) 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 Mam- 

 mal is constricted of? 



P 4-V, " TT/^11^ or, " FIG. 1228. Embryonic area of seven days' embryo Rabbit. 

 VOlK-bdC affj embryonic area ; o, place of future vascular area ; pr, 



or umbilical vesicle, so JgJ xSiikS? ; rf> medullary groove ' (From Balfour ' 

 that, ultimately, the 



two come to be connected only by a narrow yolk-stalk (Figs. 1230 

 and 1231) : 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. 



The most important of the points of difference between a 

 Mammal and a Bird, as regards the latter part of the history 

 of the development, are connected with the fate of the foetal 

 membranes. The amnion is in many Mammals developed in 

 the same way as in the Bird, viz., by the formation of a 

 system of folds of the extra-embryonal somatopleure which 

 arise from the blastoderm around the embryo, and grow upwards 



