406 BICHARD EVANS. 



reticulum begins to appear, and tlie general structure is looser 

 than in the nucleus a. The nuclei labelled d show a further 

 change, and, though they are still enclosed in a plasmodial 

 aggregation, they are so like the nuclei marked e as to be 

 indistinguishable from them, though the latter are undoubtedly 

 the nuclei of free cells which will later on develop into collar- 

 cells. An unprejudiced examination of the above figure can 

 hardly fail to satisfy the most sceptical person that the plas- 

 modial aggregations contain the nuclei of the flagellated cells 

 in a state which is only an intermediate cond,ition 

 between that which they possessed when the cells 

 were free at the surface, and tliat which later on they 

 assume as collar-cells in the interior. 



While the above proof seems conclusive, it is perhaps advis- 

 able, though it may appear superfluous, to confirm our results 

 by the comparison of several individuals preserved at diff'erent 

 stages in their development, in order to trace in them the series 

 of changes which have been described above as taking place in 

 the same pupa. In the pupa represented in figs. 16, 16 a, and 

 26 there are scarcely any nuclei or cells save those enclosed 

 iu the plasmodial aggregations and those of which the flattened 

 epithelium consists; while in that represented in figs. 18, 18 a, 

 and 18 h, which is an older pupa, the nuclei of the flagellated 

 cells are emancipating themselves at all points from the plas- 

 modial aggregations, which are losing their sharp and well- 

 defined outline. At this stage and slightly later such a thing 

 as a cell outline or limit can hardly be discerned. The nuclei of 

 the flagellated cells can be seen clearly becoming looser in 

 structure ; the linin threads are well developed ; and the 

 chromatin in general is rearranging itself preparatory to the 

 subsequent stage which is represented in fig. 19, and especially 

 in fig. 20, in which the nuclei have attained their definitive 

 characters. 



It Avould not be out of place at this juncture to lay special 

 emphasis on a fact which has been already mentioned, namely, 

 that the plasmodial aggregations always contain yolk bodies, 

 besides several nuclei. The former may vary considerably in 



