448 TREADWELL. [Vol. XVII. 



amount of ectoderm arising from these migrating cells in 

 Amphitrite is comparatively very small. 



Among the mollusks, I believe that the evidence at our dis- 

 posal sustains the theory. In Crepidula the peculiarities of 

 the 8-cell stage are probably to be explained by the presence 

 of abundant yolk in the macromeres, but the fact that D is no 

 larger than A, B, or C is due to the slow development of the 

 characteristic products of that cell — the shell gland and the 

 mesoblast. In Unio the principle certainly applies. Unfortu- 

 nately, we know too little about the later larval stages of other 

 mollusks, whose cell lineage has been studied, to draw any very 

 positive conclusions. In Umbrella (No. 14), D is smaller than 

 A, B, or C. Heymons did not follow the history of the de- 

 scendants of this cell very carefully, but describes the shell 

 gland as arising very late. If analogy can be depended upon 

 at all, this gland arises from 2d and our theory holds. 



The application of this principle to later stages demonstrates, 

 as I believe, its correctness. To mention but a single example : 

 Perhaps the most noticeable feature of the development of the 

 cross in Podarke is the way the anterior arms divide up into 

 very small cells, while the cells of the posterior ones remain 

 large. This is, I believe, correlated with the development of 

 the large, thin-walled dorsal ectoderm, while the ventral ecto- 

 derm of the umbrella is much thicker and gives rise to the eye- 

 spots and the frontal (excretory .'') bodies, the former having a 

 very much greater superficial extent than the latter. It is 

 probable also that some cells of the latter are thrown off and 

 absorbed. 



Equal cleavage, then, is not, I believe, the expression of a 

 lack of differentiation in the ovum and early embryo, but is due 

 to the fact that a certain amount of formative material, which 

 in other genera is sufficient, when accumulated in a single cell, 

 to increase noticeably the size of that cell, is here present in 

 so small a quantity that no size differences appear. This 

 small amount of formative material is correlated either with 

 the small size of the organ to which it gives rise, or with the 

 late development of that organ. 



