110 BIOLOGICAL LECTURES. 



are formed from each product of qd, they are very small, 

 " scarcely larger than polar globules," and make up but an 

 insignificant portion of qd. 



Among the molluscs the same principle applies. We have 

 already seen that it was the study of Unio which led Lillie to 

 an enunciation of the law ; and, from the results of his work 

 on Crepidttla, Conklin 1 has stated that " the initial size of the 

 blastomere stands in direct relation to the size and time of 

 formation of the part to which it gives rise." Crepidula, indeed, 

 shows apparent exceptions, for the entodermal portion of ^d 

 is as large as the mesodermal. Here, however, there is a 

 larval mesoblast, and the mesoderm derived from qd is, I 

 infer from Conklin's figures, comparatively small in amount. 

 In the earlier stages, also, 2d is no larger than its sister cells 

 of the same quartette ; but from this cell arises the shell gland, 

 which, according to Conklin, "appears late in the development." 

 Other exceptions are Umbrella? according to Heymons, and 

 Cymbulia* (Fol), where D is the smallest cell of the four-cell 

 stage ; but I believe that careful study would show that here, 

 as in the other mollus'cs, this small size is correlated with a 

 slow development of organs. Certain it is that in Umbrella, at 

 least, where, if we can trust analogy at all, the shell gland should 

 arise from 2d, this organ develops very slowly when compared 

 with such a form as Unio. Unfortunately, too little is known 

 of the later stages to enable us to draw any very certain 

 conclusions. 



Even in molluscs, where yolk appears to play an important 

 part in determining the size of cells, the constancy in the 

 arrangement of this yolk seems to me an indication that it 

 is distributed according to the needs of the embryo, and is 

 related to the greatest amount of protoplasm. If, for example, 

 all the yolk could be removed from the egg of Umbrella without 

 interfering with the normal development, I venture to suggest 

 that, while the egg as a whole would be smaller, nearly the 

 same size differences would appear as in the normal egg. In 



1 Biol. Lectures, 1896, 1897. 



2 " Zur Entwick. von Umbrella," Zeit. Wiss. Zool., Bd. Ivi. 



3 " Sur le Developpement des Pteropodes," Arch, de Zool. Exp, et Gen., 1885. 



