THE SPIRAL TYPE OF CLEAVAGE. 



263 



to be accomplished, an explanation appears possible. This divi- 

 sion is a necessary step in the concrescence of the somatic plate. 

 It does not bring " paratrochal material" into the proper posi- 

 tion or into the proper degree of differentiation, but it does aid 

 in accomplishing the end toward which each of the symmetrical 

 divisions in the somatic plate contributes its part. In another 

 case, e.g., Amphitrite, where some of the preceding divisions 

 are different and the material is 

 somewhat differently grouped, 

 no such division occurs, and 

 the paratroch consists of four 

 cells instead of six. 



The extreme precocious seg- 

 regation, which extends back 

 even to the first cleavage and 

 in some forms [Arenicola) is 

 indicated even before cleavage 

 by the shape of the Qgg (Figs. 

 26, 27), is rendered possible by 

 the fact that the whole cleav- 

 age is strictly determinate in 

 character. A determinate form 

 of cleavage is, as I believe, 

 the product of a considera- 

 ble degree of modification in 

 development in the direction 

 of condensation. Thus the ap- 

 pearance of precocious segre- 

 gation in the earliest stages must indicate a still further 

 departure from the original primitive forms of cleavage. 



Precocious segregation reaches its highest expression at 

 present in the oligochaetes and the leeches, especially in the 

 latter, where the development is very largely teloblastic, the 

 material for almost the whole body being segregated into a few 

 cells which form the germ bands. Here is not only segrega- 

 tion, but regulated cell division in the highest degree. 



Teloblastic development must undoubtedly be regarded as 

 the last term in a long series of modifications, all leading to a 



Fig. 26. — Arenicola. Egg before division, 

 seen from upper pole. 



Fig. 27. — Arenicola. Egg before division, 

 seen from side. 



