No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 279 



other three quadrants. Now in Lepidonotus there is a similar 

 dorsal interruption, and, moreover, the primary prototroch is 

 differentiated at the same time and probably in the same man- 

 ner as in Amphitrite and Clymenella (unequal type). If the 

 dorsal interruption in Lepidonotics (equal type) arises in the 

 same manner, there must be here also an early differentiation in 

 this quadrant which does not manifest itself in the size of 

 the cells. — But the origin of the co^nplete prototroch, like that 

 of the mesoderm, has yet to be ascertained in eggs with equal 

 cleavage. The ascertainment of the origin of one or both of 

 these structures is within reach, and should settle the question 

 whether one of the two blastomeres in equal cleavage is homolo- 

 gous with the larger in unequal cleavage, and the solution of the 

 last question would in turn throw light on the meaning of 

 cleavage and the nature of differentiation, morphological and 

 physiological. 



As the matter stands at present, one of two things can be 

 said of the differentiation of the egg of Lepidojtotus in the 

 64-cell stage. It has either a complete bilateral organization 

 which is not discernible, or two planes of symmetry with four 

 possibilities of orientation, from which it must select one before 

 development proceeds much farther. 



b. Unequal cleavage. 



Eggs with unequal cleavage are characterized by a 4-cell 

 stage with one cell of predominant size. This cell is the same 

 in origin except perhaps in one or two cases.* 



The destiny of the other three cells is in most cases very 

 imperfectly known. But the general homology of these early 

 blastomeres is best considered while comparing their more 

 highly differentiated products. 



All eggs of this type pass through a characteristic 8-cell 



* In Vejdovsky's account of Rhynchelmis}^ the largest cell does not correspond 

 in origin to that in the other forms, but this point needs reinvestigation (cf. 

 Whitman's criticism,^* p. 126). 



In Physa (Crampton^o) the large cell has a different origin, but has the same 

 general fate. It always forms the mesoderm, and at least the larger part of the 

 trunk ectoderm. 



