2. Larvai Stages and Fixation. 40 



and secondly that the cells of the Kentrogon which enter the body are ali alike and undifferen- 

 tiated, and not, as Delage suggested, already differentiated into ectoderm, ovary and other 

 mesodermal cellular elements. It is for this reason that I have dropped Delage's terms of 

 ovary, ectoderm etc. and substituted that of "embryonic cells" for the cellular elements of the 

 Cypris that enter the crab and give rise to the whole of the adult Sacculina. The "ovary" 

 and indeed ali the organs of the adult Sacculina are subsequent differentiations of the embry- 

 onic cells, after they have entered the crab and begun to attain to the definitive position of 

 the adult parasite. 



To the proof of these points we now turn; but a word must be said with relation to 

 the fixation in other forms of Rhizocephala. In Sacculina the fixation may be quite indefinite ; 

 at any rate the subsequent history will show us that it does not normally take place 011 the 

 underside of the abdomen where the adult Sacculina is situated. But in Peltogaster and per- 

 haps in other forms, it is possible that the fixation of the larva may take place in a position 

 near the point of evagination of the adult. For in the youngest internai stages of Peltogaster 

 which I have found, before the roots have attained any size and before any differentiation of 

 the other organs, the parasite was already situated in the abdomen of its host very much in 

 the definitive position of the adult. 



The question of the fixation indeed, whether definite or indefinite in position, assumes 

 a subordinate interest: the point which will be proved in the next chapter is that Delage was 

 perfectly correct in his idea of the discontinuity in the development from the Cypris to the 

 adult; in fact that this discontinuity is even more complete than he supposed and may be 

 best described as a kind of alternation of generations. Furthermore we will confimi Delage 

 in his statement that the passage from the Cypris to the adult is effected through stages which 

 are completely endoparasitic. 



It has been a great disappointment to me not to be able to follow out the processes 

 of fixation as described so completely by Delage, and I do not doubt that this failure will 

 be made much of by Delage's opponents; but the unprejudiced reader avìII be as little inclined 

 as I am to disbelieve Delage's minute and exhaustive account of these processes when he 

 perceives that the earliest endoparasitic stages in Sacculina, to be described in the next 

 chapter, are perfectly in accord with Delage's account of the fixation and utterly inconsistent 

 with any other method of fixation which has been suggested, or, as far as I can see, can be 

 imagined. 



I trust therefore that my failure in this matter, although it may be legitimately used 

 as a serious reproach against me, will not be employed in the service of obscurantist criticism 

 which too long has shrouded this question. 



G* 



