468 ARTHUE EOBINSON AND RICHAED ASSHETON. 



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 diagrammatically shown in fig. 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 blasto- 

 pore 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 calcula- 

 tion the fusion of layers on the lateral lips will be cut through 



