t-jg The Endoparasitic Development of the Rhizocephala. 



There is therefore a tendency towards a kind of polyembryony or budding of the 

 "nucleus" of the central tumour in Sacculina, and this led me to suspect that possibly in Pelto- 

 gaster socialis, and in such a genus as Tliylacoplethus Coutiere) in which the parasite is present 

 to the number of about a hundred (see Chap. 7), this process of budding had become normal 

 and permanent. 



According to this view it should be possible to prove that the root systems of the 

 apparently separate individuals of Peltogaster socialis on the same host are all in connection; 

 I was therefore at pains to investigate this subject. To my great disappointment I was able 

 to prove that each individual has a separate root system at no point in continuity with that 

 of another individual. Furthermore in one instance I was so fortunate as to come across a 

 crab infected with Peltogaster socialis, the individuals of which were still internal, and at a 

 very early stage in development, and even at this early stage there is no continuity between 

 the tumours and root systems of separate individuals. Plate 6 tig. 9 is a section taken through 

 two central tumours of P. socialis showing the mantle and visceral mass in process of for- 

 mation, the whole lying entirely within the body of the crab, whose tissues are tinted 

 red in the figure. The hypothesis therefore that the individuals of P. socialis are pro- 

 duced by budding from a single tumour receives no confirmation so far from the investigation 

 of facts, but I do not as yet altogether give up that hypothesis. It may well prove that the 

 splitting up of a single central tumour into the rudiments of several individuals takes place 

 at a still earlier stage, before any differentiation of the tumour has occurred, possibly soon 

 after the entrance of the embryonic cells of the Cypris larva. The final test of this hypothesis 

 must however be left to the future, for someone who has the opportunity of studying either this 

 species or some such form as Thylacoplethus, where material is abundant. 



4. The general nature of the endoparasitic development of the Rhizocephala. 



The interpretation which I am bound to give of the endoparasitic development of the 

 Rhizocephala differs, as the reader will have noticed, in an important point from that of 

 Delage. He cautiously endeavours to identify in the cells of the Kentrogon, which pass 

 into the body of the crab, the differentiated elements of the ectoderm, ovary and other meso- 

 dermal structures of the adult. But in his figure of the Kentrogon (fig. 27) these differentiated 

 elements are not distinguishable, all the cells being of the same undifferentiated embryonic 

 character; and in the fixed Cypris, a section of which 1 figure on Plate 4 fig. 20, the cells 

 of the future Kentrogon. which will enter the crab, are seen to be undifferentiated in 

 character [em . 



When we pass to the earliest endoparasitic stages described in this Chapter for 

 Sacculina and Peltogaster, which where unknown to Delage, we find that the body at this 



