FERTILISATION 225 



it is possible artificially to fertilise sea-urchins' eggs by treating 

 them with solutions of tannin and ammonia. He had already 

 formed the conception that the essential facts of cell division 

 could be resolved into a succession of processes involving 

 coagulation and liquefaction. The formation of the vitelline 

 membrane is said to be essentially a coagulative process (and 

 also possibly the formation of the centrosome and of the nuclear 

 spindle), arid the dissolution of the nuclear membrane and 

 certain of the accompanying events are regarded as evidence 

 of liquefaction. These considerations led Delage to employ 

 tannin as an agent for inducing coagulation, and ammonia for 

 causing liquefaction. Tannate of ammonia was found to pro- 

 duce a similar effect, but this is explained by Delage on the 

 assumption that, since tannin is a feeble acid and ammonia is a 

 feeble base, the ammonium tannate becomes dissociated, so 

 that the acid function (which brings about coagulation) and 

 the alkaline function (which causes liquefaction) may be sup- 

 posed to co-exist in the solution, and so separately to exert an 

 influence on different parts of the egg. By adopting the above- 

 described method, Delage succeeded in artificially fertilising 

 ova, so that they developed into complete sea-urchins, but it is 

 curious to note that the symmetry of these individuals was 

 liable to be abnormal, one of them being hexameral instead of 

 pentameral. Delage also obtained successful results by using 

 carbon dioxide and other agents, and star-fishes' eggs as well 

 as sea-urchins' were successfully fertilised. Furthermore, 

 certain of the experiments seem to indicate that the presence 

 of oxygen is not a necessary factor (as supposed by Loeb), 

 since development could be induced after practically all the 

 oxygen present had been eliminated. 



It is, of course, obvious that Loeb's interpretation of the 

 observed phenomena of fertilisation among the Metazoa is 

 inapplicable to the process of game tic union in the Protozoa, 

 in which the conjugating units are often apparently similar 

 and equipotential, and the same objection may be offered to 

 Delage's theory. It is possible, however, that conjugation in 

 the Protozoa, while presenting an essential similarity to fertilisa- 

 tion in the Metazoa, initiates a series of chemical processes of a 

 relatively simpler kind. Moreover, the theory that the changes 



