aq The Endoparasitic Development of the Rhizocephala. 



the adolescent Sacculina is evaginateci (oc). The discovery of these early stages is there- 

 fore irreconcilable with Giard's theory of locai fixation and development, at any 

 rate in Sacculina. 



The body at this stage is composed of an hollow piate which sends out processes, the 

 future roots, many of which are bifurcated and in the act of ramifying. Histologically the 

 structure is very simple; the essential facts are shown in Piate 5 figs. 3 and 4. The walls of 

 the piate are composed of large cells with nuclei which either have a conspicuous nucleolus 

 or else are in mitosis. The external wall has a layer of chitin; the wall which clings to the 

 intestine is naked, and the internai cells of this wall may be drawn out into amoeboid processes. 

 The structure of the centrai tumour (Piate 5 fig. 3 ct.cav) is the sanie as that of the roots (r) 

 and at this stage there is no sign of a differentiated ovary or organ of any kind. 



This latter fact is of the highest importance, and will be given its due prominence 

 when we consider the general nature of the development of the Rhizocephala (p. 59). 



In the next stages of development the parasite continues to throw out roots, while, as 

 a result of the growth of the whole, the part attached to the intestine begins to grow along 

 the surface of the latter towards its definitive position near the unpaired diverticulum (Piate 5 

 figs. 5 and 8). As it grows the roots become histologically differentiated into an external 

 epithelium and an internai mass of spider cells (Piate 5 fig. 18). During its passage, also, 

 the organs of the adult Sacculina first appear as a little mass or "nucleus" (figs. 5 and 8 n) 

 of cells at the posterior border of the growing parasite, that is to say at the border which 

 approaches most nearly and will soonest reach the definitive position (x). 



It is exceedingly difficult to say exactly how this "nucleus" or rudiment of the adult 

 Sacculina body arises. In the youngest stage at which I have been able to recognize it, it 

 has the appearance in section shown in Piate 5 fig. 6. This section is through the parasite 

 figured in fig. 5. The "nucleus" (ri) is seen to consist of a mass of small cells surrounded by 

 a small hollow sac (bc). The Avhole is enclosed within the walls of the centrai tumour, 

 which is composed as before of large nucleolated cells. In the cavity surrounding the "nucleus" 

 are other small cells identical with the spider cells found in the roots (sp). 



Delage believed that this "nucleus" of cells was already differentiated in the Cypris 

 larva and that the sac surrounding it (bc) is formed by an invagination of the wall of the centrai 

 tumour. It has been shown that in the earlier stages of Sacculina interna migrans the small 

 cells composing the "nucleus" do not exist as such, the only cells present being the large 

 "embryonic cells". Delage was in fact deceived when he called the cells of the Cypris which 

 pass into the crab "an ovary"; those cells become the large embryonic cells which give rise 

 to the whole mass of the Sacculina. The "nucleus" therefore is a subsequent differentiation 

 of these embryonic cells. With regarcl to the formation of the sac surrounding the nucleus, 

 I am in two minds. Delage believes that it is formed by an invagination from the wall of 

 the centrai tumour and he gives in support of this theory his fig. 36. Now I have found a 

 similar stage in which the sac (bc) is in continuity with the centrai tumour wall (Piate 5 fig. 7), 



