278 MEAD. [Vol. XIII. 



among eggs of this type. Is the larger size of the cell the 

 cause of its peculiar destiny (according to the law of progressive 

 differentiation), or is the peculiar destiny the cause of its larger 

 size (precocious segregation of embryonic material and specific 

 cell organization) "i The answer to this question depends upon 

 whether one of the two cells in equal cleavage is homologous with 

 the larger cell in unequal cleavage. This is the immediate 

 problem to be solved by the study of equal cleavage. 



In this type the two blastomeres divide into four cells of 

 equal size, the anlagen of the four quadrants. The latter can- 

 not, however, be distinguished from one another at any time 

 during the cleavage, so perfect is the quadriradial symmetry. 



Up to the 64-cell stage Lepidonotus agrees with Amphi- 

 trite in the origin and position of every cell: the rosette is 

 always formed in the same manner; the original cross cells 

 proceed to divide in the same peculiar bilateral fashion; the 

 same eight vegetative-pole cells invaginate (some divide again 

 on the surface in Amphitriie); the primary prototroch is 

 differentiated at the same time in both forms, and probably 

 from the same cells. These characters prove beyond question a 

 very high degree of differentiation in the trochophore of Lepidonotus 

 at the 6^-cell stage, even though the quadrants cannot be dis- 

 tinguished from one another. The differentiation from pole to 

 pole is shown by numerous features, — rosette, prototroch, etc.; 

 the bilateral differentiation is proved by the characteristic cross; 

 but the question remains, which of two planes is the sagittal 

 plane of the embryo? or, which of four quadrants is the dorsal 

 onef 



A clue to the solution of the problem is found in forms like 

 Amphitrite, where the dorsal quadrant is distinguished not only 

 by the size of its cells, but, after the 64-cell stage, by their 

 characteristic manner of dividing; for example, the mesoderm 

 cell, ^*, always divides bilaterally, and gives rise to the paired 

 mesoderm bands. Unfortunately, it has never been seen to 

 divide in Lepidonotus or any other annelid with equal cleavage. 

 Again, in Amphitrite and Clymenella, the interruption in the 

 prototroch is due to the failure of certain cells in the dorsal 

 quadrant to develop cilia, as do the corresponding cells in the 



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