The Development of Ischnochiton. 605 



belief that the primitive form of nervous system was, as was the 

 ancestor, radial ^). 



VI. History of the Second Quartette. 



The second quartette takes a lesser part in the development than 

 does the first or third, yet it is not second as regards the diversity 

 of its development. From its cells arise a considerable portion of the 

 stomodaeum, a part of the shell, possibly the pedal cords, certainly a 

 part of the foot and to a slight extent a portion of the body epi- 

 thelium on each side of the mouth. It arises early in the de- 

 velopment, in the 16 cell stage, and may be followed for a long 

 period. 



1. Early Cleavage and Later History. 



The cells of this quartette arise by a leiotropic division of the 

 macromeres at the same time that the formation of the trochoblasts 

 occurs and abutting against the latter they occupy furrows between 

 the macromeres (Fig. 8). The second cleavage occurs simultaneously 

 with the division of the trochoblasts and the resulting group occupies 

 its original position. This division is dexiotropic resulting in the 

 formation of a smaller superior cell {2a^ etc.) and a larger inferior 

 one {2a2 etc.. Fig. 13). 



At the next cleavage both cells are affected (Fig. 16) the upper 

 product first showing signs of division in which are formed two cells, 

 the larger upper left hand one being the tip cell of the Molluscan 

 cross, destined, except in the posterior quadrant, to furnish two cells 

 in each quadrant functioning in the prototroch ; the remaining relatively 

 slender cell enters into the "supporting layer" of the prototroch and 

 is probably to be considered in the nature of a post-trochal cell. 



From this time on the development of this quartette in the 

 posterior quadrant is along entirely different lines from the remaining, 

 and for this reason it will be omitted in the present account and 

 later considered under the history of the first somatoblast. 



As mentioned, the following division does not commence until the 

 previous one has advanced beyond its initial stages. It results in the 

 formation of a cell almost identical with the one formed in the above 



1) I have not been able to detect a nerve ring situated beneath 

 the prototroch similar to the one described by Kleinenberg for 

 Lopadorhynchus. 



