STRUCTURE OF THE LARVA OP SPONGILLA LACUSTRIS. 407 



number, and are indistinguishable from the latter at that 

 stage at which the nuclei of the flagellated cells have reached 

 the extreme limit of the modifications which they undergo 

 during metamorphosis. When, however, this limit has been 

 passed, and the nuclei in question are gradually assuming the 

 ordinary structure of those of the collar-cells, the difference 

 between them and the yolk bodies becomes more marked 

 stage by stage. Fig. 16 a illustrates a condition in which it is 

 impossible to distinguish between "yolk bodies" and nuclei|; 

 in fig. 18 this is less difficult, while in fig. 19 and still more so 

 in fig. 20 the difference, as a rule, is well marked. 



The similarity at a certain stage between the nuclei and the 

 yolk bodies makes it difficult to say whether all the flagellated 

 cells are set free from the plasmodial aggregations or not. 

 Judging, however, from certain pupae, such as that represented 

 in figs. 18, 18 a, and 18 5, there appears to be no reason what- 

 ever for the supposition that they are in any way absorbed by 

 the central cell of the plasmodial aggregation. Those bodies 

 which appear to be reduced in size, and not to be gradually 

 acquiring the ordinary nuclear characters, can be more than 

 accounted for from the number of yolk bodies occurring in the 

 larva at the time of fixation. There is no reason, however, 

 why a flagellated cell should not be completely absorbed if by 

 any mishap it was injured during the interchange of position 

 taking place at the time of fixation and subsequent metamor- 

 phosis. Any such cell would probably fall an easy prey to the 

 cells with vesicular nuclei, which in the young sponge are 

 amoeboid and nutritive in character. Nevertheless it appears 

 almost certain that the vast majority, if not all the flagellated 

 cells emancipate themselves from the plasmodial aggregations. 



The results obtained by a comparison, on the one hand, of 

 nuclei in a single individual, and on the other hand, by follow- 

 ing the diff"erent stages in several pupae, may, therefore, be 

 summarised as follows. The plasmodial aggregations contain 

 both yolk bodies and a number of nuclei. The former appear 

 to decrease in size as development goes on, and have therefore 

 supplied an argument in favour of the view — evidently incor- 



