The Development of Ischnochiton. 621 



the groups of third quartette stomatoblasts , and these grooves con- 

 tinue to deepen and to extend to other members of the second quartette 

 as the larger cells of the third quartette approach nearer to the blasto- 

 pore. For some time I thought this might be correlated with a formation 

 of larval mesoblast but it appears that this is incorrect, for the cells 

 with the exception of the stomatoblasts and possibly the adjacent cell 

 of the second quartette again come to the general level when the 

 products of the second and third quartettes are more nearly equal in 

 size. In the posterior quadrant the stomatoblast divides into two 

 cells that are small and also occupy a deep furrow that extends in 

 some cases half way up to the prototroch. No cells lie in this furrow 

 except those of the second quartette which are of small size when 

 compared with the adjacent members of the third quartette. In each 

 case these grooves appear to have no deeper significance than that 

 they are a result of a compression of small cells between much 

 larger ones. 



Comparisons. 



The earlier stages in the gastrulation of Chiton marmoratus and 

 C. squamosus have been studied by Metcalf without however deter- 

 miniûg accurately the cell lineage. This has led I believe to some 

 errors in the interpretation of certain points which may possibly be 

 corrected in the light of the development of Ischnochiton as the re- 

 lation of the cells in the two forms is almost identical. 



In the first place it is an interesting fact that the macromeres and 

 the cells corresponding to the fourth quartette appear to have been 

 formed by spiral cleavages ; that they are also remarkably small cells 

 and above all have no difference iu their size in the various quadrants. 

 Chiton is a more primitive genus in external respects at least than 

 Ischnochiton, and it would be of interest and importance to determine 

 if this uniformity of cell mass in the macromeres and fourth quartette 

 represents a primitive condition where the mesoblast is less difleren- 

 tiated from the entoderm than in Ischnochiton. 



The macromeres are first affected in the invagination. As they 

 press into the interior the cells of the third quartette divide forming 

 cells homologous with the third quartette stomatoblast. All are 

 relatively enormous products of the same size in each quadrant, and 

 possibly in the formation of the archenteron compensate for the small 

 size of the entoderm cells. Metcalf speaks of these cells (stomato- 

 blasts) as entoderm. It is true they enter into the formation of the 



