416 RICHAED EVANS. 



groups, but immediately after breaking away from them, aud 

 coming into contact with the cells with vesicular nuclei 

 situated in the interior, they undergo the structural changes 

 which have been already described. 



The peculiar arrangement of the flagellated cells in the 

 groups under consideration appears to be due to the pressure 

 exerted upon them by the cells with granular nuclei, which 

 are struggling towards the exterior at all points on the surface 

 of the individual. The two processes, namely, the passing in 

 of the flagellated cells and the passing out of the cells with 

 granular nuclei, go on at the same time, both classes of cells 

 taking an active part, apparently, in effecting the interchange 

 of position. 



The next feature to be considered is one of the greatest 

 importance, and consists in the existence of a stage in which 

 there are no free cells derived either from cell groups or from 

 the flagellated layer of the larva. The pupa at this stage 

 consists of plasmodial aggregations inside and flattened epi- 

 thelium on the outside (figs. 16, 16 a, and 26). There are no 

 signs whatever of the larval cavity. The formation of the 

 plasmodial aggregations has been so fully dealt with already, 

 that it is unnecessary to say anything further with regard 

 to them. However, it must be pointed out that the nuclei 

 of the flagellated cells sometimes — though this is not the 

 usual rule — undergo the changes which have been described 

 while still remaining in close association with the central 

 cell which possesses a vesicular nucleus. The nuclei marked 

 n. fl. c. at the left-hand corner of fig. 20 illustrate this 

 point. Further, the nuclei labelled a, b, and d in fig. 17 

 exemplify the successive stages in the series of changes 

 through which they pass. The nucleus marked a in fig. 17 

 has already increased in size, and contains the chromatin in 

 an evenly distributed condition ; while in those labelled 6, 

 linin threads of considerable thickness, covered with chro- 

 matin, are appearing. The nuclei marked d present the ulti- 

 mate structure of those of the collar-cells, though they are 

 still situated inside a plasmodial aggregation. In the face 



