600 HAROLD HEATH, 



in Crepidula the velum itself progresses but slowly. This tardy 

 development of the velum is probably due to the lengthening of the 

 pre-larval period owing to the secondary accumulation of considerable 

 yolk. While this delays the division of the trochoblasts and con- 

 sequently retards the assumption of the free swimming condition it 

 does not apparently hinder the development of the velar field. However, 

 if Crepidula were to form an accessory cell in the same way as is 

 formed in the forms considered above, and at the time when the 

 velum had reached a degree of differentiation represented by twa 

 cells in each quadrant, the development of the cross would be greatly 

 retarded, and instead of there being 42 cells in the cross there would 

 be in all probability not more than 12 or 16. But since the de- 

 velopment of the cross is so precocious, or probably better since the 

 differentiation of the velum is so slow, I believe the cleavage that 

 normally should form the accessory cell is omitted. 

 Whether the omitted cleavage ever occurs or is in other words post- 

 poned is a question that at present cannot be solved ^). 



It is interesting to note that in the anterior arm of Crepidula 

 plana, arising possibly as a product of the basal cell, are two small 

 cells that degenerate and it is believed are ultimately pushed out and 

 thrown away. The thought suggests itself that these two degenerating 

 cells represent products of the accessory cell but there is nothing to 

 prove this is actually the case. 



From the foregoing description it will appear that a cross such 

 as occurs in Ischnochiton and Umbrella is present in the Annelids 

 though it does not become apparent owing, for one reason at least, 

 to the small size of the basal cell; also the cross in these forms is 

 the same as that in Crepidula. This latter animal lacks a leiotropic 

 division that is present in the other forms, but after these have 

 passed through that cleavage their position and history, so far as this 

 has been traced, is similar. 



d) Later History. Commencing with the stage as represented 

 in Fig. 23 the spindles of the basal cells lie perfectly horizontal and 

 each resulting cleavage forms two cells of equal size. In very many 

 cases I have noticed that the posterior basal cell divided slightly in 

 advance of the others, yet this appears to be of little significance as 

 precocious cleavages of the daughter cells are rather rare at the next 

 division. 



1) Cf. page 642. 



