function. Their evenly granulated cytoplasm and their superficial 

 position would both favour this view. 1 



The thesocytes in Spojigilla contain a large vacuole filled with sub- 

 stance of an amyloid nature, and in addition a certain number of solid 

 amyloid grains. The presence of these substances is perhaps due to the 

 activity of the chlorophyll corpuscles which the cells contain. For an 

 account of their nature and their reactions to stains, etc., see Lankester 

 (1882). 



The thesocytes probably correspond in part to the cellules sphe'ru- 

 leuses, a name by which Topsent seeks to distinguish a class of cells 

 found in all sponges, and frequently containing bodies of amyloid nature, 

 representing reserve nutriment. The possession, however, of " spherules," 

 i.e. of large refringent granules, is not one sufficient of itself to dis- 

 tinguish a class of cells. Topsent's cellules spheruleuses are certainly 

 porocytes in Ascons, and are probably the same in many other cases. 

 In some cases, however, they may represent thesocytes, i.e. trophocytes 

 charged with reserve materials. Loisel [10] has shown that in Reniera two 

 classes of cellules spheruleuses occur : (1) isolated cells containing nutrient 

 amyloid bodies ; (2) cells within which are formed the elastic fibrils. 

 The former, in our opinion, would be thesocytes, the latter porocytes. 



The three possible differentiations of the amoebocytes or wander- 

 ing cells would therefore be ingestive cells or phagocytes, nutritive 

 cells or trophocytes, and finally, storage cells or thesocytes. It is 

 probable, however, that any wandering cell can perform each or all 

 of these functions, and that the characteristics by which one or 

 another of the different kinds of cells can be distinguished are 

 of transitory nature, and mark simply a passing phase of the 

 metabolism. 



In addition to the large wandering cells, there occur in Ascons others 

 of excessively minute size, not more than four or five p in length, each 

 with a minute, faintly staining nucleus and clear cytoplasm. They 

 often occur in nests, as if they had originated from the breaking up of 

 larger cells, and it is possible that this is the manner in which the 

 ordinary wandering cells reproduce themselves in these sponges, and that 

 each of these minute cells is destined in its turn to grow into an ordinary 

 wandering cell. Their complete history is not as yet made out, but this 

 view receives some support from the fact (1) that cells are commonly to 

 be found showing every gradation of character intermediate between 

 these minute cells and the ordinary wandering cells ; and (2) that the 

 numerous small cells produced in the ontogeny by breaking up of the 

 posterior granular cells pass in the young sponge into the condition of 

 these minute wandering cells (Fig. 58, 5, ara.c). 



(/3) Tokocytes. From a purely histological point of view the re- 

 productive cells may be regarded as a form of thesocyte, a tropho- 



1 Tt is not impossible that Fiedler's phagocytes might be simply porocytes (cellules 

 spheruleuses). Cf. footnote to p. 49, supra. 



7 



