264 MEAD. [Vol. XIII. 



the same way and from the very same cells as in Amphitrite 

 (Figs. 80, 81, 83, 85, 86, text Fig. VI). 



The lowest cells A^, B^, C^, D^ (64-cell stage) divide once 

 more on the surface as in Amphitrite, resulting in a definitive 

 entoderm plate of eleven cells. The position of these, however, 

 is slightly different : a* and a^ border upon A^ in Clymenella, 

 while only a^ does so in Amphitrite. The same arrangement 

 obtains in the quadrants b and c. A^, B^, and C^ each develops 

 a large "vacuole," which can be followed for a long time after 

 the invagination. 



Figs. 78-88 show that the divisions of the somatic-plate cells, 

 and those of the other ectoderm cells, «°"^ <^^"^, c^'^, and a^, F, c^, 

 as far as they have been followed, agree closely in the two 

 forms. The paired mesoderm cells M and M divide about 

 equally upon the surface, while in Amphitrite this division is 

 very unequal (p. 248).* 



The trochophores of the two annelids have a general resem- 

 blance, though Clymenella is less active. 



Summary. — Though the &%% of Clymenella is much larger 

 than that of Amphitrite, and though the worms belong to 

 entirely different families, the similarity in the cleavage is very 

 remarkable. 



Up to the 64-cell stage all the cleavages correspond in 

 direction, and within certain limits the relative size of the 

 cells is the same. In the later stages, there is still a wonderful 

 resemblance even in details ; the formation of the rosette, 

 cross, and prototroch, and the division of the somatic plate up 

 to eleven cells at least is identical. The entoderm Q) and the 

 mesoderm (.'') is derived from the same cells in both forms. 



The axial relationships as far as followed are the same in 

 both annelids. 



Clymenella differs from Amphitrite in the absolute size of the 

 Q.g%y in the relative size of the anterior and posterior hemi- 

 spheres, the larger size of d'' (mesoderm cell), the earlier 

 division of M and M on the surface, the relative size of the 

 products of this division, and the smaller size of X"^. 



* Of course in calling these cells entoderm and mesoderm, I am presuming that 

 their destiny as well as their origin is like that of other forms. 



