4IO 



WILSON. 



[Vol. VI. 



6%, represent the thirteenth. The residual teloblasts continue 

 to divide in the same manner for some time, but finally break 

 up into small cells indistinguishable from the remaining cells 

 of the ventral plate. The latest stage in which I could certainly 

 distinguish them is shown in Fig. 70, but in considerably later 

 stages (Figs. 81, 82) a group of larger cells can still be seen on 

 either side, which is probably the product of the teloblast, and 

 marks the site of its disappearance. During these divisions the 

 residual teloblasts move apart so as to leave a triangular space 

 between them covered with small transparent cells {dor), and at 

 the same time they gradually recede from the prototroch toiuards 

 the lower pole. This change of position in the teloblasts is of 

 fundamental importance, since I believe the triangular area to 

 represent the middle dorsal region of the adult body, and the 

 residual teloblasts to mark the posterior limit of the ventral 

 plate. Under this interpretation, therefore, the recession of the 

 teloblasts towards the lower pole represejits the shifting or bend- 

 ing of the a7iteroposterior axis of the larva tozvards its definite 

 position in the adult body. This view is fully discussed at p. 431. 



With the disappearance of the teloblasts the history of the 

 first somatoblast may be said to close, and I will here only call 

 attention to the side and ventral views of the various stages 

 shown on PI, XIX. The later history of the ventral plate will 

 be briefly touched upon further on, but I shall defer to a future 

 paper an account of the histological differentiation. 



The following diagram gives a general view of the divisions up 

 to the disappearance of the residual teloblasts : — 



I \ 



r X - X - X- X 



/ \ 



X - X - X - X \ 



\ 



*2 



^ X - X - X - X \ 



I / 



x^ x^ 



