THE SPIRAL TYPE OF CLEAVAGE. 



245 



native is the correct one, it seems clear that the spiral form of 

 cleavage and precocious segregation are very closely connected 

 in annelids and mollusks. 



In the forms under consideration — forms possessing spiral 

 and determinate cleavage — there is imposed upon the spiral 

 cleavage proper a strictly bilateral form of cleavage, in which 



Fig. 9. 



Fig. 10. 



Fig. 



Fig. 12. 



• Fig. 9. — Arenicola. Somatic plate consists of three cells which have arisen by spiral divisions. 

 Fig. 10. — Arenicola. Somatic plate after spiral divisions have ceased. 

 Fig. II. — Are7iicola. Somatic plate. The cell dp gives rise to the two dorsal cells of the para- 



troch, the cell xl \.o the two lateral paratroch cells of the left side. 

 Fig. 12. — Arenicola. Somatic plate. The cell a^ dividing spirally. 



the axes of symmetry are the same as in the adult {cf. Fig. 5 

 with Figs. 7 and 8, also Fig. 9 with Fig. 10). This bilateral 

 type of cleavage is evidently connected with the process of 

 morphogenesis, and I desire to call it, for convenience, a 

 nioi'pJiogenetic cleavage. A morphogenetic form of cleavage 



