6o 



SPONGES 



cyte in which the absorptive or anabolic power is increased, the 

 distributive or katabolic function largely in abeyance. When 

 special trophocytes exist, the tokocytes in their earliest stages 

 resemble them in all points, and undoubtedly belong to this class of 

 cell elements. 



In sponges generally two classes of tokocytes can be dis- 

 tinguished : first, sexual cells or gonocytes, the mother cells of ova 

 and spermatozoa of the normal type ; secondly, gemmule cells or 

 statocytes, such as compose the gemmule in Spongilla. 



The gemmule cells will be discussed when considering the growth 

 and development of the gemmules ; it is sufficient here to say that they 

 arise from the same stock as the sexual cells, and that both in appearance 

 and potentialities they are comparable in every way to blastomeres of 



Fie. 55. 



Sperm cells of sponges, a-h, development of spermatozoa of Sycon raphanus, x 792 ; h, 

 mature spermatozoa (after Polejaeff) ; j, a sperm ball in Oscarella lobularis, x 500 ; k, a mature 

 isolated spermatozoon (after Schulze), x 800. 



the segmenting ovum. We may consider more especially the origin of 

 the sexual cells. 



The spermatogenesis has been studied in a number of forms, and 

 appears to conform to one of two types. In the first type of sper- 

 matogenesis, which has been especially studied by Polejaeff in Sycon, and 

 by Fiedler in Spongilla, the male gonocyte or spermatogonium undergoes 

 a division of the nucleus into two dissimilar nuclei, one of which travels 

 to the periphery of the cell, while the other remains near or at the centre 

 (Fig. 55, a and 6). The protoplasm then segments off in connection with 

 the peripheral nucleus to form a covering cell or spermatocyst surrounding a 

 sperm mother cell or spermatocyte. The former may remain single (Sycon), 

 or may divide again to form two covering cells (Spongilla). The sper- 

 matocyte undergoes repeated cell division by karyokinesis to form a 

 number of spermatids, each of which becomes a spermatozoon in the 

 usual way, the nucleus giving rise to the head, the cytoplasm to the tail. 

 The result is a mass of spermatozoa or sperm ball, enclosed by a covering 

 cell (Fig. 55, c, d, e, f, g). The second type of spermatogenesis is essentially 

 similar, but differs in the absence of any covering cell, the whole sper- 

 matogonium giving rise to a sperm ball, which may be enclosed in an 



