STRUCTURE OF THE LARVA OF SPONGTLLA LACUSTRIS. 371 



nuclei appear to pass out iudividually over the whole surface, 

 and when they have got to the exterior they flatten out and 

 become soldered together by their margins to form the layer 

 of flattened epithelium which covers the surface. 



The flagellated cells in all cases, after passing into the 

 interior, enter into the formation of peculiar associations in 

 which no cell limit can be seen except just at first. These 

 associations, the " polynuclear groups" of Delage, will be 

 described in this paper asplasmodial aggregations (figs. 

 16, 16 a, 29, 29 a and b). More than half the mass of the 

 bodies in question must be made up in many cases of the 

 flagellated cells which have entered them by a process of active 

 migration, and which later on separate themselves from them 

 in the same manner. 



The nuclei of the flagellated cells undergo peculiar modi- 

 fications. They contract, and the chromatin becomes re- 

 arranged and increases in quantity, so that by the time this 

 change has reached its extreme limit the nuclei of the flagel- 

 lated cells are very difficult to distinguish from the "yolk 

 bodies." In each plasmodial aggregation the external 

 limit of the cell mass as a whole is at one time sharp and well 

 defined ; but this condition is transitory, lasting but a short 

 time, and is therefore not often met with. The plasmodial 

 aggregations become ill-defined and run into one another, 

 thus presenting a syncytium-like arrangement, in which it 

 is most difficult to make out cell boundaries. During this 

 period the nuclei of the flagellated cells change in their cha- 

 racters, and pass by degrees into a condition which represents 

 the definitive state of the nucleus of the collar-cell; their 

 framework becomes looser in texture, owing to the appearing 

 of threads and ultimately of small irregularly shaped granules, 

 and a well-defined nuclear membrane is formed. The nuclei 

 of the flagellated cells, while undergoing these changes, are as 

 a rule arranging themselves in rings in the cytoplasm round 

 cavities of a circular shape, and in each such ring the cyto- 

 plasm becomes divided into cell bodies corresponding to the 

 nuclei. The cells so formed are independent of one another, 



