300 THE CELL 



we can definitely prove that the sperm-nucleus, or, as is more 

 frequently the case, the sperm-nuclei, which have penetrated into 

 one of the fragments of the ovum, become metamorphosed into 

 small typical nuclear spindles with a radiation at each pole. The 

 sperm-nucleus now splits up into daughter-nuclei, which for their 

 part again multiply by indirect division, so that the fragment of 

 the ovum breaks up into a number of small, embryonal cells. 

 Boveri (VIII. 2) has pursued this observation further, and has 

 discovered the important fact, that out of a rather large non- 

 nucleated fragment of an ovum, which has been fertilised by a 

 single spermatozoon, a normal, although proportionately small, 

 larva can be developed. 



b. Apogamy. The phenomena, which de Bary (VII. 2) has 

 included under the name of apogamy, have a close relationship 

 to parthenogenesis, and may be conveniently treated now. 



Apogamy has been observed in certain Ferns ; it is well known 

 that in the course of their development there is an alternation of 

 generations. Minute plants, the prothallia, are derived from the 

 vegetative reproductive cells, or spores ; the function of these 

 prothallia is to develop male and female sexual organs, the latter 

 of which produce egg-cells. These, when fertilised, produce an 

 asexual Fern-plant, which develops spores in a vegetative manner. 



In Pteris cretica and Asplenium filix-femina cristatum and fal- 

 catum, the law of alternation of generations, which is generally so 

 constant in Ferns, is broken through. The prothallia of these 

 three species either produce no sexual organs at all, or only such 

 as are no longer functional, i.e. have become rudimentary; on the 

 other hand, a new Fern arises from the prothallium by means of 

 vegetative budding. 



Since these three species of Ferns have been affected by culti- 

 vation, it is possible that the development of cells requiring 

 fertilisation has been suppressed by excessive nourishment, 

 whilst the vegetative mode of reproduction has been favoured. 



2. Sexual Affinity. By sexual affinity we understand the re- 

 ciprocal influences which are exercised by cells of related species 

 requiring fertilisation upon each other. This takes place in such 

 a manner, that, when the cells are brought within a definite dis- 

 tance of one another, they exert a mutual attraction upon each 

 other, and combine, fusing into one, like two chemical bodies, 

 between which unsatisfied chemical affinities existed. If both 



