FORMATION AND FATE OF THE PRIMITIVE STREAK. 97 



extends on one side, and the line from ZZ ends at the furthest point 

 from the blastoporic lip to which the fusion of those layers extends on 

 the other side. It will be noticed that on one side the fusion extends 

 further than on the other. No doubt this is chiefly owing to the section 

 being not quite horizontal, the side of the ZZ being the more ventral 

 of the two. We have, however, taken the thicker side for the purpose 

 of measurement. 



If the diminution in size of the blastopore has been produced by the 

 concrescence of the ventral portion of the lateral lips as suggested above 

 and as diagram matically shown in figure 10, it is clear that the fusion 

 of layers ought to extend further from the actual lip of the blastopore 

 on the ventral border of the blastopore than on the sides or dorsal 

 border. This will be clearly seen by reference to the diagram, fig. 10. 



It is equally clear that this would not be so if the closure of the 

 blastopore were strictly centripetal, in which case the fusion of layers 

 should extend on all sides equally. 



If the distance be measured across the blastopore in fig. 7 with a 

 pair of compasses, it will be found to be about the same distance as 

 between the edge of the blastopore lip and the point of furthest 

 extension of fused epiblast and mesoblast on the side of ZZ. 



In the series of sections from which this fig. 7 and the two 

 represented in figs. 8 and 9 were selected, the open blastopore was 

 present in forty sections. 



Assuming the open blastopore to have been circular, we may 

 conclude that it would have required forty sections to cut through 

 the fused mass between the edge of the blastoporic lip (B) and the 

 furthest extension of the fused mass indicated by the line drawn from 

 ZZ', for we have just now seen that the distance from B to B is about 

 the same as from B' to ZZ. In other words, the forty-first section 

 ought to show no, or but little, fusion of layers. 



Fig. 9 has been drawn from the forty-first section, and instead of 

 there being no fusion of layers there is an extent of fused layers about 

 equal to the two fused blastoporic rims, or indeed, a little more. 



At the sixtieth section the epiblast and mesoblast are still distinctly 

 fused, and it is not until the eightieth section below the edge of the 

 ventral lip of the blastopore that the layers can be definitely said to be 

 separate. In short, whereas by calculation the fusion of layers on the 



G 



