XIII 



PHYLUM CHORDATA 



589 



the trophoblast, known as the covering layer or Banker's layer, 

 lying over this embryonal ectoderm, has a widely different fate 

 in different Eutheria : it may thin out and disappear. 



A primitive knot and embryonic shield are formed as in Reptiles. 

 The primitive knot has simply the appearance of the somewhat 

 enlarged anterior extremity of & primitive streak (Fig. 1216, 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 latter, viz., active 

 proliferation ot cells leading to the development of the begin- 

 nings of the mesoderm. A dark median streak, the head- 

 process, appears in front of the primitive knot, and in some 



FIG. 1216. Embryonic area of a seven days' embryo Rabbit, ag, embryonic area ; o, place of 

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

 Kolliker.) 



Mammals there is an invagination on the surface of the former 

 leading to the formation of a neurenteric canal and of a noto- 

 chordal 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 underlying space 

 such as occurs in Reptiles (p. 3G5). A medullary groove (rf) and 

 canal are formed in front of the primitive streak, and a row 

 of protovertebraa (Fig. 1217) make their appearance on each 



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