290 MEAD. [Vol. XIII. 



When Lang's figures and text are examined in the light of 

 this comparison, the correspondence of Discoccelis to the other 

 forms is complete in every detail: so complete as to be fairly- 

 startling. The eight cells are formed in the same manner 

 (Figs. 12-16). The one which corresponds to the mesoderm 

 cell in the other animals, divides bilaterally, and one product 

 lies on either side of the middle line (Fig. 17, «„ a^, PI. XXXV, 

 text Fig. XXV, p. 340). 



Since in a large series of forms the mesoderm cell is the 

 same in origin (same generation and position), and immediately 

 divides in a manner different from all the other cells, and since 

 a cell in Discoccelis corresponds exactly in origin and begins 

 the same characteristic career, I believe it may be the mesoderm 

 cell in Discoccelis also. 



In many cases the paired mesoderm cells divide while still 

 at the surface. The ventral or "anterior" products, whose 

 exact fate is uncertain, may be as large as the posterior 

 (Clymenelld), or much smaller, as is usually the case {Nereis, 

 Unio, Umbrella, Spio, Aricia, Crepiduld). In Spio and Aricia 

 Wilson says the anterior products "are very minute and appear 

 to be rudimentary." In AmpJiitrite they are extremely minute, 

 arising at a later period by a cleavage which seems to defy 

 mechanical conditions, and are carried into the segmentation 

 cavity and lie at the anterior ends of the mesoderm bands. In 

 Polymnia this "preliminary superficial budding seems not to 

 take place." 



We cannot interpret this peculiar division until more is 

 learned about the exact fate of these cells. The fact that they 

 are sometimes of large size and divide readily, while at other 

 times they are much smaller or exceedingly minute and are 

 slow in dividing, would seem to indicate that they are the 

 anlage of some variable or rudimentary structure. 



Entoderm. — The cytogenetic origin of the entoderm is 

 another example of cell homology. In many forms the larger 

 of the four cells at the vegetative pole in the 32-cell stage 

 divides, the outermost product giving rise to the mesoderm cell, 

 while the rest of the products are entodermal, whether they 

 divide again on the surface or not. In Nereis the entoderm 



