624 HAROLD HEATH, 



anterior to the blastopore and the resultiog plate occupies this 

 position it is difficult to understand by what method the nerves form 

 from the plate and occupy a posterior position. It scarcely seems 

 probable that such profound changes occur in Ch. polii and not in 

 Ischnochiton nor in any of the Annelids and Molluscs thus far described. 

 I would suggest that owing to the difficulties in the way of the ob- 

 servation of the embryos of Chiton polii ^ Kowalevski has wrongly 

 oriented the earlier stages. Accordingly the so-called anterior groove 

 and later tube are probably structures posterior to the blastopore. 

 In some cases in Ischnochiion development the posterior third quartette 

 stomatoblasts do meet on the median ventral line as in Fig. 39 but 

 the tube in such cases is very short and soon disappears, not by a 

 fusion of its walls, but by a species of evagination in which the cells 

 forming the bottom of the groove come out level with the ecto- 

 dermal cells of the surface of the body. So in Ischnochiton the walls 

 of the tube never fuse and the mouth never completely closes, and if 

 these processes do occur in Chiton polii they are inexplicable in the 

 light of the development of any nearly related groups. 



Gastropods. The accurate study of the formation of the 

 stomodaeum has not been made in any Mollusc hitherto, and con- 

 sequently comparisons are not possible beyond the earlier stages. In 

 Crepidula during the earlier stages of invagination the second and 

 third quartettes have the same general relations and many of the 

 cell cleavages during this time are identical to those in Ischnochiton. 

 But what part these play in the formation of the stomodaeum is unknown. 



Annelids. In the light of many other resemblances between 

 the embryos of Nereis and Ischnochiton it is impossible to believe 

 that the similarity of the vegetative poles in these two forms is 

 accidental. The blastopore is quadrangular in both and the secondary 

 stomatoblasts lie in the angles ; the third quartette composes its sides ; 

 and within the space thus enclosed are the macromeres. On the 

 posterior side of the Nereis embryo modifications have arisen but it 

 is still easily possible to compare these with similarly located 

 structures in Chiton. These secondary changes, consisting primarily 

 in the excessive development of the posterior second quartette cells 

 (first stomatoblast) are responsible for the changed position of the 

 cells, the tertiary stomatoblasts becoming widely separated, yet they 

 remain in the same relative position as in Ischnochiton. 



As the ectodermal cells bounding the blastopore converge toward 

 a central point as they do in Ischnochiton, the tertiary stomatoblasts, 



