STEaCTUEE OF THE LAEVA OP SPONGILLA LACUSTUIS. 401 



irregularity disaj)pears (figs. 24 and 24 a). When the marginal 

 membrane has become so thick as to consist of two or three layers 

 of cells, there often appears a space between these layers, and 

 into this space flagellated cells, accompanied either by cells 

 with granular nuclei or by cells with vesicular nuclei, find their 

 way. The nuclei of the lower layer of cells, constituting the 

 marginal membrane, display the same difference of size from 

 those of the upper layer as was described above as existing 

 between those of the lower and upper surfaces in general. The 

 clearly defined limit characteristic of its cells at an earlier 

 stage is now no longer visible, and the extreme margin 

 presents in all cases a regular and unbroken edge. The mar- 

 ginal membrane seems to be nothing more than a continuation 

 of the dermal epithelium of the upper and lower surfaces, into 

 the inner portion of which the internal substance of the 

 sponge enters comparatively late, never passing into the outer 

 margin, which later on is retracted. 



The Changes through which the Flagellated Cells 

 pass at Fixation. — The passage of the flagellated cells to 

 the interior has been already described, but it must not be for- 

 gotten that there is considerable variation in the rate of change 

 of position, which affects, in its turn, the time necessary for 

 the complete enclosure of the cells in question in the interior 

 of the pupa. It often happens that they have completely dis- 

 appeared from the lower surface, while they still remain as a 

 fairly complete layer on the upper. 



The flagellated cells after passing to the interior undergo a 

 most extraordinary series of changes, during which the nuclei 

 seem to lose their internal structure so completely that at one 

 stage they are almost indistinguishable, except by chemical 

 reactions, from the yolk bodies. The changes in question have 

 already commenced in the larvae represented in figs. 15 a and 

 29 b, where a few cells are seen adherent to the surface of 

 some of the cells of the inner mass, i. e. tct cells with vesicular 

 nuclei, and in some cases even to cells with granular nuclei 

 as well. The first sign of the disappearance of the ordinary 



