96 CONK LIN. [Vol. XIII. 



The cross in Umbrella develops more slowly than in either 

 of the other forms, — thus the basal cells are formed at the 

 24-cell stage in Neritina, the 25-cell stage in Crepidula, and the 

 39-cell stage in Umbrella. The first division of the basal cells 

 occurs at the 3 /-cell stage in Neritina, the 44-cell stage in 

 Crepidula, and the 83-cell stage in Umbrella. On the other 

 hand, the terminal cells are formed when 25 cells are present 

 in Umbrella, 28 in Neritina, and 30 in Crepidula. Likewise 

 the division of the turret cells, which occurs at the 63-cell 

 stage in Umbrella, does not occur until long after the iii-cell 

 stage in Crepidula, at which point I ceased to follow the lineage 

 of the entire ^^g. But in spite of these two cases in which 

 Umbrella outstrips Crepidula, the division of the cells of the 

 first quartette is much slower in the former than in the latter. 

 Thus there are in Umbrella at the 91 -cell stage 16 cells of the 

 first quartette ; in Crepidula, at a corresponding stage, 23 

 cells. In both cases the greatest activity is in the second 

 quartette. Heymons says of Umbrella : " The micromeres of 

 the first generation are smallest, those of the last largest " {gen- 

 eration is used in the sense of quartette). In Crepidula the 

 differences are not marked, though I think the second is some- 

 what larger, when formed, than either the first or third. The 

 larger size and more rapid division of the cells of any quartette 

 are probably connected with the larger size or more rapid devel- 

 opment of the organs to which they give rise, as Lillie ('95) has 

 established in the case of Unio. The velar field (derived from 

 the first quartette) is certainly larger and develops more rapidly 

 in Crepidula than in Umbrella, and corresponding to this we 

 have the larger size of the cells when first formed and their 

 more rapid divisions subsequently. The smaller size of the 

 velar field in Umbrella may account for the relative unimpor- 

 tance of the cross in that animal. Concerning the fate of the 

 cross cells in Umbrella nothing is known. 



Heymons has observed, but does not figure, the division of 

 the terminal cells and a second division of the basal cells ; the 

 direction of these divisions is not given. He has also observed 

 the rosette division by which four small cells are formed at the 

 apical pole, " strikingly like the apical rosette of Wilson " ; as 



