290 DR. J. B. GATENBT ON THE GERM-CELLS AND 



Spermatogenesis *. 



1. The spermatozoon is probably a (flagellated) ovoid body, containing a 

 bun-shaped nucleus. The spermatid is flagellate, so we know that the 

 spermatozoon must be flagellated, though one was never found outside a 

 sperm-carrying cell. In PI. 19. tig. 6 is drawn a sperm from a sperm- 

 carrying cell, the tail being added from observation of spermatids. The 

 sperm has an acrosome in front, which lies on the edge of the cell, but which 

 is not pointed. The nucleus of the spermatozoon is bun-shaped, reticulate, 

 and contains a nucleolus (karycsome) ; the mitochondrial apparatus is in the 

 form of a bun-shaped fuchsinophile, macromitosome (" Nebenkern "), or 

 middle-piece. The centrosome lies partly between the nucleus and macro- 

 mitosome, but more in the latter than in the former (PL 19. tig. G). 



2. Spermatogenesis takes place inside a definite capsule of pale, cubical, 

 granular, non-flagellated mesogleal cells. The sperm-stages are aggre- 

 gated into little patches here and there throughout the entire area of 

 the hermaphrodite sponge ; these patches lie beneath the flagellated 

 gastral epithelium, and stain more darkly than their surroundings. Both 

 spermatocytes and spermatids have been identified ; the latter are flagellated 

 (test-fig. 1 and PI. 19. fig. 5 a). 



3. The number of spermatids in one patch is generally above fifty. 



Fertilization (1st Part). 



1. The ripe sperm, after having entered into the gastral cavities, is carried 

 from the collar-epithelium to the oocyte by means of a carrier-cell. The 

 sperm lies inside the carrier-cell, which bears its passenger to the oocyte. 



2. The carrier-cell is assumed to be a modified collar-cell or choanocyte, 

 for the following reasons : — 



((() There are two cases in which I have found a sperm inside a collar- 

 cell (text-fig. 2, PI. 20. fig. 10). 



(b) There are a large number of cases in which I have found a sperm 



inside a cell which closely resembles a collar-cell, except for absence 

 of collar and flagellum, and which lies still within the collar- 

 epithelium in the position drawn in text-fig. 3. 



(c) There are about fifty cases in which I found the spermatozoon 



contained within a cell which alwaj's lay in the region indicated by 

 the black dots in text-fig. 4. No sperm-carrying cells or spermatozoa 

 were found in any part of the mesoglea or at the short sides of 

 the egg. 



(d) I have never found, nor has any other author figured, mesogleal 

 amoebocytes lying near unfertilized oocj'tes, in the invariable 

 position in which the sperm-carrying cell later appears — only 

 collar-cells are found there ; the gastral cavities were not found to 

 contain sperm-carrving cells — the collar-cells alone can be the 

 source of the sperm-carrying cells. 



* See foot-note on page 279. 



