STRUCTURE OF THE LA.RVA OF SPONGILLA LACUSTRIS. 405 



granules, and the threads of the nuclear reticulum become 

 visible. The chromatin granules are smaller and stain faintly. 

 They are more numerous than they were in the primitive con- 

 dition of the cell, and the nuclear reticulum is not so coarse 

 (figs. 17—20). 



In the description of the larva given above reference has 

 been made to a slight change in the character of the nuclei of 

 the individual elements of the cell groups, taking place as they 

 pass from the condition of structure found in type B to that 

 which exists in type C. It was pointed out that the nuclei 

 of the cell groups in type B are indistinguishable from those 

 of the flagellated cells at the surface. As a matter of fact, 

 the nuclei of the collar-cells in type C are equally indistinguish- 

 able from those of the flagellated cells at the period when 

 the latter — by their own energy, apparently — emancipate 

 themselves from the plasmodial aggregations into which they 

 entered, and again assume their individual form. Here, there- 

 fore, is another argument, in addition to those already brought 

 forward, in favour of the view that the flagellated cells at the 

 surface and the cell groups in the interior are homologous, 

 and really belong to the same class of cells. 



As it is impossible to observe the above changes in the living 

 pupa, we are forced to the method of studying them in sections, 

 and so drawing our conclusions. In our examination of sections 

 we may follow one of two courses. It happens sometimes 

 that the changes in the structure of the nuclei of the flagel- 

 lated cells, as they pass from the condition in which they are 

 difficult to distinguish from the yolk bodies to that found in 

 fully developed collar-cells, can be traced while they are still 

 inside the plasmodial aggregations, — in other words, the 

 changes can be followed in one individual, or even in one 

 section of such an individual. 



Fig. 17 illustrates the changes which go on in these nuclei 

 as they pass from one condition to the other, and shows a 

 number of transitions in one individual. The nucleus marked 

 a has become enlarged, and the chromatin is scattered almost 

 uniformly in it; while in the nuclei marked b the nuclear 



