STKUCTURE OF THE LARVA OF SPONGILLA LAOUSTEIS. 387 



which have a tendency to protrude through the flagellated layer, 

 though they are still completely surrounded by the cells which 

 produced them. The various changes undergone by the cells 

 with vesicular nuclei suffice, apart from any other reason, to 

 prove that the order of the types above adopted is the true one 

 as regards age : i.e. that type A is the youngest of all, because 

 the interior of the solid posterior end of the inner mass is 

 monopolised by the large cells with vesicular nuclei ; and that 

 type B must be younger than type C, because the characters of 

 the remaining cells with vesicular nuclei are almost the same as 

 the characters of those cells in type A, though the numerical 

 proportion which they bear to the other cells has changed con- 

 siderably (figs. 11, 11 a, 12, 13, and 13 a and b). 



The individual elements of the cell groups which have been 

 described in type B as being incompletely divided, and as 

 having their nuclei lying near the periphery of a common 

 mass of cytoplasm, are, in this type, completely independent of 

 one another. The cell groups now are as numerous as they 

 were in their incipient condition in type B, a state of things 

 markedly different from what exists in the type of larva still 

 to be described (figs. 7, 7 a, 11, and 11a). The cells which 

 constitute the groups are fairly uniform in size. They 

 possess a nucleus which measures about 2^^ in diameter, 

 showing a slight increase in size as compared with the nuclei 

 found in the incompletely divided groups of type B, as well as 

 a greater number of chromatin granules and a higher degree 

 of complexity in the nuclear reticulum. Sometimes, however, 

 there may be seen in a group one cell larger than the others 

 with a nucleus of corresponding size. The existence of such 

 cells is probably to be explained as being the result of inde- 

 pendent growth after the cells have become liberated from the 

 incompletely divided groups of type B (fig. 11 a). 



The number of cells which constitute a group varies con- 

 siderably, just as the number of nuclei vary in the multi- 

 nucleated masses of cytoplasm in type B. There may be no 

 more than four or five cells in a radial section of a group, or 

 there may be a dozen or even more, but it is probable that the 



