270 PR- •!• B. GATENBY ON THE GERM-CELLS AND 



noticed that the constant position of entry of the sperm may only be due 

 to the fact that the gastral side is the nearest path which the sperm could 

 take to the egg. 



The fertilized egg soon loses that temporary polarity conferred by the 

 disturbing influence of the spermatozoon, and soon the granules so dis- 

 arranged flow back and become evenly disposed (PI. 21. figs. 18 and 19). 

 No methods used by me succeeded in demonstrating any differences in 

 cells of the two halves of the early blastula (PI. 21. fig 21), and it should be 

 mentioned that in several cases it seemed to me that the future flagellate 

 hemisphere, instead of being turned towards the nearest gastral cavity, was 

 turned away from it, as drawn in PI. 22. fig. 27, where I presume the 

 spermatozoon entered near the letters SPX. 



The conclusion which seems most likely to be correct is that the oocyte 

 lies with its animal pole towards the gastral cavity, and that the sperm 

 enters at this pole ; one may also suppose that the animal pole gives rise 

 to the flagellated cells or histocytes, while the vegetative hemisphere gives 

 rise to the granular cells. It may be noted, too, that the oocyte cytoplasm 

 shows, on a microscopical basis, no signs of regional differentiation. 



I believe that while the latter remark is correct, it may be true that the 

 ground-plasma of the oocyte is more or less perfectly differentiated into 

 two sorts of substances, which may be the forerunners of the two different 

 categories of cells of the amphiblastula larva. 



17. Preliminary Outline of Events leading to the Union of Sperm and Egg. 



The following account of the stages leading to the union of sperm and 

 ego- is derived only partly from assumption, but mainly from actual observa- 

 tion of microscopical preparations. 



The spermatozoa are carried into the gastral cavities, presumably being- 

 borne along by the inhalent currents. Passing to that region of the gastral 

 epithelial carpet overlying a ripe oocyte, one spermatozoon enters a collar- 

 cell (PI. 19. fig. 9). After the entry of the sperm into the choanocyte or 

 collar-cell, the latter loses its collar and flagellum, becomes amoeboid, and 

 gradually passes through the basement-membrane of the collar-cell epithelium 

 (PI. 20. fig. 14), and penetrating all obstacles finally comes to lie on what I 

 have tentatively presumed to be the animal pole of the egg : generally the 

 sperm-carrying collar-cell makes a special depression on the surface of the 

 oocyte. Protoplasmic continuity is established between oocyte and sperm- 

 carrying cell, and the spermatozoon is passively borne into the egg by 

 means of protoplasmic currents (PI. 20. fig. 12). 



In short, this account would mean to say that the differentiated flagellated 

 collar-cell may become modified, in a short space of time, into an amoeboid 

 non-flagellated cell, whose function is to carry the spermatozoon to the egg. 



