ix MOLLUSCA 333 



quite easily preserved. For the earlier stages corrosive sublimate 

 and picro-sulphuric acid were the reagents used, but for the later 

 stages and for the free-swimming larvae Hermann's mixture of 

 osmium tetroxide, platinum chloride, and acetic acids gave the 

 best results. It was necessary to paralyse the larvae by cautiously 

 adding cocaine to the water in which they lived, before attempting to 

 preserve them, otherwise they contracted themselves into shapeless 

 lumps in which the natural relationship of the various organs 

 could not be made out. The larval stages swarmed in the 

 lake and were captured by using a fine-meshed Plankton net, so 

 that the difficulties connected with artificial rearing were entirely 

 avoided. 



A striking feature of the early development of Dreissensia is the 

 intermittent appearance of the blastocoele. This cavity is large 

 and well developed in the 2-cell stage (Fig. 263) ; it subsequently 

 disappears, but reappears in later stages, 

 such as the 8-cell and 16-cell stages. 

 Meisenheimer supposes that the blasto- 

 coele serves as a reservoir of excreta 

 which are periodically voided. 



The egg divides into the usual four 

 macromeres A, B, C, and D, but of 

 these D is so much larger than the rest 

 that the remaining three appear much 

 like micromeres budded from one large FIG. 263. -Longitudinal section of 



macromere. This State of affairs is the 2-cell stage of Dreissensia 



worth bearing in mind in view of the poiymvrpim to show the biasto- 



, . , . , , eoele. (After Meisenheimer. ) 



extraordinary statements which have 



, J . .,-,-, w - c > blastocoele ; n, nucleus. 



been made about the development of 



other Pelecypoda. When the first quartette of micromeres is formed, 

 Id appears first and is the largest, though the disparity in size 

 between it and its sisters, la, Ib, and Ic, which appear subsequently, 

 is not great. But at the next cleavage, when these micromeres divide, 

 each into two daughters of equal size, and when the second quartette 

 of micromeres is formed, one of these latter, 2d, is relatively enormous 

 in size ; it overshadows not only all the micromeres belonging to the 

 first and second quartettes but its own sister macromere, 2D. This 

 huge " micromere " corresponds to the one which Wilson, in the 

 development of Nereis, has termed the first somatoblast, from which 

 most if not all the ectoderm covering the body of the adult worm is 

 derived. 



From the first somatoblast of Dreissensia, termed by Meisenheimer 

 X, is derived the shell-gland, and we have strong reason to suggest, 

 although this is not quite proved, the foot; it has, however, been 

 proved by Lillie (1895) in the case of Unio. The first somatoblast 

 now gives off a cell below and to the right. This cell is of course 2d 2 : 

 it is denominated by Meisenheimer x 1? since he calls the parent 

 somatoblast X. At the next cleavage all the daughters of the first 



