1904.] NATURAL SCIENCES OF PHILADELPHIA. 341 



Umbrella, although Heymons' conclusion regarding the fate of the 

 descendants of 4d is at wide variance with the conditions which are 

 found in Fiona. After the cleavage of 4d into equal parts, Heymons 

 states that two small cells are given off from these, so that they lie in 

 the posterior region of the macromeres. It is very evident from his 

 figures that these cells, which would correspond to E^, E^ of Fiona, 

 at first lie quite dorsal to the enteron and in the cleavage cavity. The 

 large cells next divide nearly equally, the most posterior being slightly 

 smaller and corresponding in size and origin to m^z^, m-z^. These 

 latter shortly change their position in Umbrella exactly as in Fiona, 

 for, says Heymons, ''Bald beginnt eine interessante Lagerungsver- 

 schiebung einzutreten. Es rucken namlich die hinteren Zellen weiter 

 nach dem animalen Pol hin und legen sic vollkommen auf die vorderen 

 auf". While this rearrangement is occurring and after its completion 

 two and later other small cells are given off by the large underlying 

 cells toward the smaller cells originally budded forth. Exactly the 

 same process occurs in Fiona — compare Heymons' figs. 23 and 24 with 

 my fig. 71. Heymons' smaller cells M', M' (corresponding to m*z^, 

 m^z^ of Fiona), which have moved toward the animal pole of Umbrella, 

 do not appear from the account to divide again so quickly as 

 in Fiona, but that they later divide teloblastically is evident. 

 As has been mentioned before, the small anterior cells of Umbrella, 

 which correspond to E\ E^, e\ e^ of Fiona, at first lie entirely within 

 the segmentation cavity. Figures of later stages, however (Heymons' 

 fig. 29), show that they then lie at a level with the posterior cells of the 

 enteron (D, A', C, etc.), and are directly between these and the anal 

 cells. The same relative position is taken by the corresponding cells 

 of Fiona. 



In interpreting the results of Heymons the above point of view is 

 somewhat different from the comparison of Conklin between Umbrella 

 and Crepidula, in which he suggests a resemblance and possible simi- 

 larity of origin between the enteroblasts of Crepidula and the telo- 

 blastic cells M, M, M', M', of Umbrella. In both these "are large cells 

 containing a considerable quantity of yolk, about equal in size and 

 grouped in a characteristic way" ; but the same may be said of the 

 similar cells of Fiona, yet they have no part whatever in the formation 

 of the enteron, though from their appearance I was led to think such 

 might be the case before a knowledge of their later history proved 

 otherwise. The explanation of the whole matter lies in the axial 

 change which the derivatives of 4d have undergone in the forms con- 

 sidered. The posterior macromeres (particularly D) of Umbrella are 



