1. The History and Development of Sacculina interna. £ | 



composing the wall of the central tumour in this region of invagination take on a more 

 definitely epithelial character. In figs. 12 and 13 (Plate 5) the invagination has proceeded 

 further, and an arm is beginning to enclose the Sacculina's visceral mass on its outer side. 

 In Plate 5 fig. 14 the perisomatic space (pc) is completely formed and surrounds the visceral mass 

 and the mantle cavity altogether. All this time the walls of the inner cavity, i. e. the mantle 

 cavity or brood pouch (be), have remained unaltered and have taken no part in the formation of 

 the perisomatic cavity, which has been formed entirely from the wall of the central tumour 

 by an invagination proceeding more rapidly in the region lying between the Sacculina and 

 the crab's intestine. Delage makes his perisomatic cavity at first closed and then opening 

 secondarily (Text fig. 12); according to me the perisomatic space is open from the beginning. 



During the formation of the perisomatic space or cavity, the rest of the organogeny 

 has been proceeding. The first organ to appear, besides the brood-pouch, is the ovary 

 (ov Plate 5). This is distinguishable as a large mass of cells surrounded by a membrane; 

 it is bilobed so that in many of the sections there appear to be two bodies, as in Plate 8 

 figs. 12 and 13, but where a section passes through either one of the lobes or the intermediate 

 portion of the ovary uniting the two lobes, the mass appears single (fig. 11). Surrounding 

 the ovary at this stage are a mass of indifferent cells which pass insensibly into the filamen- 

 tous spider-cells (sp) which fill the central tumour (Plate 5 figs. 11, 12 and 13). 



Certain of these spider-cells intrude between the outer epithelial layer of the brood-pouch 

 (fig. 13ep2) and that layer of the central tumour which will form the inner wall of the perisomatic 

 space (fig. 13ep3). These cells form the circular musculature of the outer wall of the brood-pouch. 

 The transverse musculature, i. e. the fibres which stretch across the thickness of the outer wall 

 of the brood-pouch, I agree with Delage in deriving from the epithelial cells of the central 

 tumour (Plate 5 fig. 13ep3) and from the outer epithelial layer of the brood-pouch (fig. \"&ep2). 



The mesentery (mes) is formed during the invagination of the perisomatic space. 

 Ever since the appearance of the central "nucleus" and its investing mantle sac, a portion of 

 the "nucleus" of cells has been left unenveloped by the mantle sac (fig. limes). As the epi- 

 thelium of the central tumour rolls in to form the perisomatic space, it unites with the cells 

 of the central "nucleus" to form the mesentery along the line where the mantle cavity is 

 blocked (fig. 1 1 mes). 



The formation of the nervous ganglion (Plate 5 figs. 13 and llgri) takes place when 

 the perisomatic space is nearly completed. The nervous ganglion pushes into the visceral 

 mass from the outer epithelial layer of the mantle-sac [be), opposite the extreme anterior 

 region of the mesentery and slightly to one side of the middle line. This position will be 

 more clearly explained on p. 53. 



The testes (t) are not apparent until a rather late point in development. Two little 

 pockets push in from the summit of the mantle sac to form the vasa deferentia, and grouped 

 round them two masses of cells with rather large nuclei can be distinguished, which will 

 give rise to the glandular portion of future testes (Plate 5 figs. 1 5 and 1 6, £ and dv) . 



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