484 ARTHCJE ROBINSON AND RICHARD ASSHETON. 



which figs. 15^ 16, 17 were drawn, the actual fate of the 

 primitive streak in this region may be fairly easily deter- 

 mined. 



In comparing fig. 20 with fig. 15, the change that has 

 taken place in the former seems to be of this nature. The 

 three layers, epiblast, mesoblast, hypoblast, instead of being 

 " fused," are now (fig. 20) easily distinguishable and com- 

 pletely apart. 



This fusion of layers we must most probably interpret as 

 indicating an area of proliferation of cells, which indeed is a 

 characteristic feature of a primitive streak ; and when this 

 proliferation of cells ceases, the primitive streak may be said 

 to be no longer of physiological importance, though the area 

 of its former extension, being of morphological interest, should 

 be noted. 



Thus, if we are regarding the primitive streak from a 

 physiological point of view, we must say that this portion has 

 now ceased to exist. If we regard it from a morphological 

 point of view, we may say that this portion has ceased to be 

 " functional," but, nevertheless, includes the area in the 

 bracket labelled PS in fig. 26 as being within the limits of the 

 original extension of the primitive streak. This portion we 

 have attempted to render more distinct in fig. 26 by a different 

 method of shading and crossed lines, PS' PS'". 



The fate of the ventral moiety may be understood by 

 reference to fig. 26, combined with a comparison of figs. 20, 

 31, 22 with figs. 15, 16, 17. By words we may explain its 

 fate by saying that the ventral moiety of the primitive streak 

 splits up into portions of the three germinal layers. 



(I) Laterally and ventrally into (a) the posterior ex- 

 tremity of the mesodermal plate. 



(II) In the median plane into two layers, (b) an outer or 

 epiblastic layer, forming part of the skin, and (c) an inner or 

 hypoblastic layer of large darkly pigmented cells, fig. 20, HY', 

 forming the hind wall of the rectal spout. 



The difference in character of cell between the posterior 

 wall and anterior wall of the rectal spout is very marked, as 



