HYBRIDITY 37I 



seen that the egg requires a developmental stimulus and we may suppose that 

 the sperm-cell by itself, perhaps only on account of its poverty in protoplasm, 

 perhaps also for other reasons, is incapable of development. Bearing on the 

 first of these possibilities is the phenomenon of ' merogony ', which has been 

 examined in both animals and plants (RosxAFiNSKi ; WINKLER, 1901). By 

 appropriate means, portions of the egg containing no nuclei were detached 

 and fertilized. The sperm-cell when embedded in a considerable mass of proto- 

 plasm commences to develop. This shows that the sperm-cell when enclosed 

 in protoplasm derived from the egg-cell becomes capable of development and 

 undergoes ' andro- ' or ' ephebo-genesis ' ; we may indeed give it another meaning 

 and say that the nucleus of the ovum is unnecessary to development, and that 

 it is sufficient if a sperm-nucleus is added to a non-nucleate egg. 



Moreover, there is also a question of fundamental importance to be faced, 

 viz. why are the two sexual-cells incapable of development on their own account ? 

 Is the inhibition of development due to internal causes ? Is it a result of old age 

 and is the fusion to be regarded as a case of rejuvenescence? This latter hypothesis 

 has often been suggested, although not proved, though sound arguments 

 against it are not forthcoming. We must content ourselves therefore with a 

 a reference to what has been said already against the need for rejuvenescence, 

 adding only that the means employed is extremely peculiar. It is certainly 

 by no means clear how by the fusion of two senile units a rejuvenescence can 

 result one might just as well expect that such a fusion should lead to increased 

 senility. If we can allot a more suitable meaning to the fusion, then the idea 

 of rejuvenescence as resulting therefrom may be readily given up. 



In recent years another interpretation of the process has become more and 

 more prominent and has now received very general support, namely, that the 

 fusion of the two cells in the act of fertilization is of primary importance, inas- 

 much as it unites the characters of two organisms. In asexual reproduction, 

 i. e. by means of spores in Algae and Fungi, a cell is set free from the mother- 

 plant, and from this a new organism is produced with characters similar to 

 those of the parent. The spore thus transmits the characters of the ancestor 

 to the offspring, the latter inherits the ancestral peculiarities. If now in 

 two plants or in two branches of the same plant differences of some kind exist, 

 then these differences at least under certain conditions may be transferred 

 to the offspring. In asexual reproduction, which we may describe as monogeny, 

 individual peculiarities may thus remain unaltered. Let us now assume that in 

 sexual reproduction (digeny) each cell carries potentially certain individual differ- 

 ences derived from its parent, or, as it is usually put, has in it- initials of these 

 characters, then these initials are united and mixed in the fertilized egg. Hence 

 we may, with WEISMANN (1892 b), term fertilization 'amphimixis'. So far, 

 there is a general concurrence of opinion amongst authors that this union of 

 characters is the predominant feature in fertilization, but whether the signifi- 

 cance of the union be the balancing of individual characters or whether by a blend- 

 ing of the two organisms new characters originate is still a matter of discussion. 



We must now attempt to discuss these two possibilities more closely, and 

 first of all it may be emphasized that in such an interpretation of fertilization 

 the observed inhibition in the development of the sexual-cells at once attains 

 a new significance. It is to be regarded as an adaptation, which in the first 

 instance renders fusion possible. For if the egg or sperm surrounded itself with 

 a cell- wall immediately after its formation and then began to grow, any fusion 

 of the protoplasms and nuclei of the two cells would be impossible. 



We must not, however, overlook the fact that the above views do not 

 harmonize with all the facts we recognize under the term fertilization. If the 

 swarmspores formed by division in one cell of an alga copulate in pairs on 

 swarming free from the mother-cell (p. 354), the differences between these cells 

 can hardly be so great that a mixing has any very special significance. The 



B b 2 



