No. 3] THE CELL-LINEAGE OF NEREIS. 431 



entoblast, and only secondarily by the subsequent displacement 

 of its products. This represents the opposite extreme to Limi- 

 briciis, where the mesoblast retains its original position. Lopa- 

 dorhynchus occupies a middle ground. The series seems to me 

 an interesting example of the reaction of external mechanical 

 conditions on the form of cell-division. 



The problem of inheritance which it involves is touched on 

 in a following passage. 



We may now briefly consider the ventral plate. The essen- 

 tial points in its history are as follows : At first the entire mate- 

 rial of the ventral plate (first somatobiast) lies in the median 

 line, quite behind the mesoblast (V, A). By the division of 

 the somatobiast into two, and ultimately into four, equal parts, 

 this material is equally distributed on either side the middle 

 line, though the two halves subsequently become so intimately 

 united along the middle line that they can only be distinguished 

 posteriorly. Finally, as the body elongates, the two halves of 

 the ventral plate grow forward at the sides of the pigment-area 

 (cf. Diagram V, A, B), inwards between it and the stomodaeum, 

 and undergo a process of concrescence along the median ventral 

 line, as described at p. 422. The pigment-area meanwhile is 

 carried backwards. (The pigment-area might be said to migrate 

 backwards, virtually cutting its way through the middle of the 

 ventral plate. The primary cause of the change seems, how- 

 ever, to lie in the growth of the ventral plate.) 



Meanwhile the shifting of the neural axis takes place. Men- 

 tion has been made at p. 410 of the recession of the residual 

 teloblasts from the prototroch. This, I conceive, is caused by 

 the increasing development of the dorsal region, which has 

 hitherto remained in a rudimentary state, and it marks the 

 beginning of the rotation of the neural axis, as shown in Dia- 

 gram V, B. I have not been able to follow the recession fur- 

 ther than the stage indicated in the diagram ; but when the 

 facts of LopadorhyncJms are recalled, it is impossible to doubt 

 that the region of the residual teloblasts is ultimately carried 

 down to the region of the pigment-area.^ The final condition 



1 This view of the axial shifting is essentially in accordance with the earlier view 

 of Hatschek, which he now seems inclined to abandon in favor of the view that it is 

 only the mouth and stomodaeum that undergo a change of position (^Zoologie, III, pp. 

 319, 320). 



