FORMATION AND PATE OF THE PRIMITIVE STREAK. 489 



tube has become separated from the skin, and with it the dorsal 

 portion of the primitive streak. 



The primitive streak should, however, still be connected 

 with the skin ventrally by that portion which does not become 

 folded up and nipped off as does the dorsal portion, together 

 with the neural tube. 



Possibly this connection may exist for a very short time, 

 but practically the separation from the skin of the dorsal 

 moiety takes place contemporaneously with the splitting up of 

 the ventral moiety, so that the space between the dorsal moiety 

 of the primitive streak and the skin from which it has separated, 

 and the space between the posterior wall of the rectal spout 

 and post- anal gut on the one hand, and the skin at the same 

 level caused by the splitting up of the primitive streak of that 

 area, become confluent at the point in fig. 26 just where the line 

 PS" crosses the space between primitive streak and skin. 



Thus it comes about that the dorsal portion of the primitive 

 streak, which remains " functional " and gives rise to the 

 greater part of the tail, comes to lie entirely within the 

 embryo. 



Fig. 26 represents the stage at which the tail has definitely 

 begun to grow. The dorsal moiety of the primitive 

 streak is seen to be lying within the embryo, and it is 

 by proliferation of its cells that the whole of the tail 

 is formed with the exception of the skin. In no section 

 after (the stage shortly antecedent to this) have we been able 

 to see a fusion between the skin and the primitive streak, 

 though the skin lies closely over the primitive streak. 



The skin would seem to grow, not at any one point, but over 

 its whole surface, on account of the pressure caused from within 

 by the growth of the main axis of the body, in response to which 

 the skin must either grow or rupture. 



To a certain extent the skin of the middle line of the ventral 

 surface of the tail is derived from the primitive streak ; that is, 

 from the ventral moiety by the splitting up of the latter. This 

 portion is distinguished in fig. 26 by a different mode of shading; 

 but with the exception of this, which we think extends only a 



