36? 



LECTURE XXIX 

 FERTILIZATION, PARTHENOGENESIS, HYBRIDITY, HEREDITY 



[In dealing with these phenomena we enter on a subject which, in spite of its great interest 

 to physiologists, has been investigated almost solely from the morphological point of view. 

 This renders its discussion very difficult as well as not infrequently onesided and subjective, 

 since the main problems still await decision by experiment. In the additional notes in the 

 present English edition only a selection of the recent literature can be dealt with.] 



In plants, as in animals, there is, as we have already seen, a characteristic 

 method of reproduction by the fusion of two previously separate cells, and in ex- 

 treme cases the fusing cells, like the organs which produce them, are so widely 

 different from each other that we may then speak of sexual differentiation and 

 of the ' fertilization' of a female by a male cell, in harmony with the conditions 

 existing in the higher animals. In endeavouring to arrive at the meaning of 

 fertilization we may consider the process as it occurs in definite cases among 

 Archegoniatae and Angiospermae without inquiring whether fertilization has 

 the same value in all cases. Since, doubtless, the sexual process has arisen not 

 once only, but several times in the history of organisms, we must look on it as 

 possible that in different places it may have a different significance, e.g. in 

 Diatomaceae and Phanerogams (compare Klebs, 1899). 



In certain Algae the principal difference between egg-cell and sperm lies 

 in the much greater size of the former ; but both the gametes are normal cells, 

 each has protoplasm and a nucleus, and each may even possess chromatophores. 

 But in Phanerogams and ferns nearly all the protoplasm of the male cell dis- 

 appears and the cell consists finally of little more than a nucleus. The 

 protoplasm is never entirely absent, but, as far as we know, in the higher plants, 

 the chromatophores are always wanting. We may therefore assume that the 

 nucleus of the sperm is a most important organ, and is responsible for the final 

 results of fertilization. 



During the process of fertilization the protoplasts of the two cells unite and 

 the two nuclei also undergo fusion, and thereafter the fertilized egg begins to 

 grow. Without fusion neither male nor female cell is capable of growth. 

 Thus the first result of fertilization is the removal of a developmental inhibition, 

 and we may conclude that the egg was in want of something which is supplied 

 by the sperm, and that its nucleus is destitute of certain substafices which the male 

 nucleus possesses. 



It is probable that further research may show a qualitative difference to 

 exist between the two nuclei, although at present such differences have not been 

 determined. On the contrary research has hitherto shown a similarity in the 

 materials composing ovum and sperm nuclei, and amongst these nuclein has 

 been regarded as most worthy of attention. Although Zacharias (1901) in 

 general found no qualitative difference in the nuclein present he found a con- 

 siderable quantitative difference — the ovum-nucleus being much poorer in 

 nuclein than the sperm-nucleus. In certain cases, for instance Marchantia, this 

 difference becomes qualitative for no nuclein at all can be detected in the egg- 

 nucleus just previous to fertilization. On this has been founded the hypothesis 

 that the ripe ovum is incapable of development because it does not contain sufficient 

 nuclein, a deficiency which is supplied on fertilization ; but that the sperm cannot 

 develop by itself because it is deficient in protoplasm. 



Views entirely different from that just advanced are held, two of which may 

 be briefly referred to here ; one is based on the phenomena exhibited by the 

 nucleus of the cell preceding the formation of the sperm and ripening of the 

 ovum — phenomena resembling those seen in animals. It has often been re- 

 marked that the formation of male and female cells begins with a nuclear 

 division differing widely from the typical mode, and described as reduction- 



