PHVLITRI CHORDATA 



089 



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

 lying over this embryonal ectoderm, has a widely different fate 

 iu different Eutheria : it may thin out and disappear. 



Aprimidre knot and enibri/onic shiclJ are formed as in Reptiles. 

 The primitive knot has simply the appearance of the somewhat 

 enlarged anterior extremity of ajirimitwe streak (Fig. 121G, 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 h'eail- 

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



Fin. 1:216.— Embryonic area of .a .seven days' embryo Rabbit, "r/, enibryunic area ; o, place u£ 

 future vascular area ; ^yr, primitive ytreak ; //, medullary gro(.>ve. (From Balfour, after 

 KoUiker. ) 



Mammals there is an invagination on the suiface of the former 

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

 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 sjjace 

 such as occurs in Reptiles (jx 305). A medullary groove (rf) and 

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

 of protovertebrai; (Fig. 1217) make their appearance on each 



VOL. II u o 



