SPONGES 



83 



took the view of Ganin, but later adhered to that of Delage, except as 

 regards the phagocytosis. Finally, Nuldeke agrees with Delage that the 

 flagellated cells are ingested in a phagocytic manner by cells of the inner 

 mass, but believes them to be then completely absorbed, the whole sponge 

 developing, as Gutte supposed, from the inner mass alone. 



The careful investigations, recently published, of Evans [34] show 

 that, as might be expected from a comparative survey of sponge embryology, 

 the Hagellated cells of the larva do furnish the collar cells of the adult, but 

 that they may be supplemented in this function by other cells of the larva 

 in a very interesting manner. In the inner mass there are always to be 

 found large granular cells, similar both in appearance and potentialities to 

 blastomeres of the segmenting ovum or to cells of the gemmule, and marked 

 out by containing a large amount of reserve food material (nutritive 

 vacuoles and yolk-granules). These cells are to be regarded as archaeocytes, 

 which are able to give rise to tissue cells of any kind ; while, on the one 

 hand, their destiny, so long as they remain unmodified, is probably to be- 

 come the amoebocytes of the adult, they may, on the other hand, in their 



Five stages in the developinent of a Hagellated chamber from a blastomere in the inner inass 

 of Spongilld. 1, a blastomere and two cells of the inner mass ; 2, the nnclear corpnscle of the 

 blastomere has broken up into a number of chromatin bodies within the nnclear membrane ; 3, 

 the nucleus of the blastomere has become fragmented ; 4, the small nuclei so produced have 

 arranged themselves at the periphery of the cell, the cytoplasm of which is beginning to show 

 lines (if cleavav'e between them ; 5, the original blastomere has broken up into a number of 

 collar cells, arranged in a chamber ; the two cells of the inner mass form part of the epithelium 

 of the excurrent canal. Slightly schematised. (After Evans.) 



capacity of reproductive cells (tokocytes) contribute towards either the 

 dermal or the gastral layer. In the latter case they undergo a sort of 

 fragmentation, affecting first the nucleus and then the cytoplasm, and 

 resulting in the formation of a number of small cells, which, even during 

 the larval period, arrange themselves to form a Hagellated chamber, each 

 cell acquiring the characteristic collar and flagellum (Fig. 64, 1-5). The 

 histological composition of the inner mass varies greatly, even in the 

 larvae of one and the same species of fresh-water sponge ; in some specimens, 

 chambers, in even their incipient stages of development, are almost or 

 entirely absent from the inner mass ; in others they occur abundantly and 

 in various stages of formation. In the latter case the Hagellated layer 

 of the larva is perhaps jiartly absorbed at the metamorphosis, and the 

 chambers of the adult are derived chieHy from those of the inner mass of 

 the larva. In short, the development of Spnvfiilla may take different 

 courses in different instances, the end result being, however, the 

 same in all case.*. The wav in which the chambers and other tissue- 



