FORMATION AND PATE OF THE PRIMITIVE STREAK. Ill 



all three germinal layers are now distinct and separate. In fig. 22 the 

 mesoblast ME may be said to be still fused with the epiblast EP'", 

 but it is nevertheless, quite distinct in character of cell, and in degree 

 of pigmentation. In the two other sections, figs. 21 and 20, the mes- 

 oblast has become entirely separated from the epiblast. 



By comparing these three sections with the sections from which figs. 

 15, 16, 17 were drawn, the actual fate of the primitive streak in this 

 region may be fairly easily determined. 



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 completely 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 impor- 

 tance, though the area of its former extension, being of morphological 

 interest, should be noted. 



Thus, if we are regarding the primitive streaks 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, 21, 22 with figs. 15, 16, 

 1 7. 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 extremity of the 

 mesodermal plate. 



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

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

 layer of large darkly pigmented cells, fig. 20, HY\ forming the hind 

 wall of the rectal spout. 



