EARLY DEVELOPMENT OF GRANTIA COMPRESSA. 277 



I consider that I have found all stages confirming this peculiar happening, 

 not only in Grantia, but also in part in Sycon ciliatum. Jorgensen's 

 figures (8 a), misinterpreted, show that the same process takes place in 

 Sycandra raphanus. 



18. Evidence as to the True Identity of the Sperm-carrying Cell. 

 In by far the greater number of cases, the earliest stages of fertilization 

 that I have found are those at a time when the sperm-carrying cell has 

 reached the egg (PL 20. figs. 11 and 14). In many such cases it is quite clear 

 that the nucleus of the sperm-carrying cell has a somewhat clearer sub- 

 stratum, and may be bigger than that of the collar-cell. Did one know only 

 these special cases, one might be led to consider that the sperm-carrying cell 

 was some sort of amcebocyte from the mesoglea. Nevertheless, I have con- 

 cluded definitely that the sperm-carrying cell is, or was, a collar-cell, and 

 for the following reasons : — 



1. In certain few but definite examples the ripe spermatozoon has been 

 found in a collar epithelial cell, which reposed in place alongside its fellows 

 (text-fig. 1, camera lucida). 



2. In a very large number of examples I found spermatozoa in cells in the 

 position indicated in text-fig. 3 — that is to say, still in the epithelium, but 

 more or less covered by its neighbours. 



3. In a very large number of cases, I believe the majority, the nucleus 

 and the cytoplasmic inclusions of the sperm-carrying cell exactly resembled 

 those of the collar-cell or choanocyte, while the size of the former approxi- 

 mated closely to that of the latter (see page 279). 



4. The position of the point of entry of the sperm-carrying cell into tissue 

 of the sponge, on its journey to the oocyte, is remarkably constant (compare 

 text-fig. 4) ; yet when one examines ripe oocytes before the advent of the 

 spermatozoon, no special cells can be found in the epithelium at the region 

 where the sperm-bearing cells later appear. 



These reasons lead me to believe that the sperm-carrying cell is a modified 

 choanocyte ; I consider that any one of the statements given in the above 

 four paragraphs is sufficient to indicate the true nature of the sperm-carrying 

 cell, and all the facts together constitute a clear and indisputable proof. 



The occasional slight change in the density of the nuclear substratum 

 (linin) I consider explicable under the assumption that the presence of the 

 spermatozoon exerts an influence leading to this change, and it is important 

 to note that this change does not always occur. 



The matter is further discussed on page 283. 



19. Behaviour of Sperm after Entry into Collar-cell. 

 In my own preparations I never found the last stages in spermatogenesis, 

 so that it is impossible for me to say whether the nucleus of the sperm is 

 altered by its entry into the collar-cell. In IT. 19. fig. 5 A, I have drawn 



LINN. JOUEN. — ZOOLOGY, VOL. XXXIV. 21 



