THEORY OF FERTILISAIION. 159 



that contact of the male fluid with the ovum was absolutely 

 necessary. Even he, however, went away h'om the true con- 

 clusion, by maintaining that the fertile male fluid of toads was 

 destitute of spermatozoa. That the above vague conceptions 

 have been replaced by the certain conclusion, that intimate 

 cellular union is the sine qua nou of fertilisation, we have already 

 emphasised. 



i:^ 2. Modern T/ieo7'ies of Fertilisation — Morphological. — 

 Recent investigators of the facts of fertilisation have generalised 

 their results in different ways according to their dominant bias. 

 Some mainly restrict themselves to stating the morphological 

 facts, and to emphasising the relative importance of cell-sub- 

 stance and of nuclei in the union ; others attack the deeper 

 problem of the physiological import of the process, — a problem 

 the full solution of which is still remote ; while others have 

 confined themselves rather to discussing the uses of fertilisation 

 in relation to the species. Some representative positions on 

 each of these planes must be sketched ; and, first of all, the more 

 morphological theories, and the very important question whether 

 the union of nuclei is everything, or whether the union of cell- 

 substance has also its import. 



{a) Herhoig's Vie7v. — Professor O. Hertwig, who was one of the first 

 carefully to follow out the details of fertilisation in animals, thus sums up his 

 " TJieorie der BefnicJifnng" : — "In fertilisation, distinctly demonstrable 

 morphological processes occur. Of these the important and essential one 

 is the union of two sexually differentiated cell-nuclei, the female nucleus of 

 the ovum and the male nucleus of the sperm. These contain the fer- 

 tilising nuclear substance, which is an organised substance, and acts as such 

 in the process. The female nuclear substance transmits the characters of 

 the mother, the male nucleus those of the father, to the offspring." The 

 nucleus is thus the essential element both in fertilisation and in inheritance. 

 {h) Slrashurge7-''s Vieiv. — What Hertwig maintains for animals, 

 Strasburger does for plants. "The process of fertilisation depends upon 

 the union of the sperm nucleus with the nucleus of the egg-cell ; the cell- 

 substance (cytoplasm) does not share in the process." "The cell-sub- 

 stance of the pollen-grain is only the vehicle to conduct the generative- 

 nucleus to its destination." It may become nutritive, he allows however, 

 to the germ-rudiment. " Generally the uniting nuclei are almost perfectly 

 alike," though there may be slight differences in the size of the nucleoli. 

 "The two cell-nuclei do not differ in their nature, they are not sexually 

 differentiated in the ways that the individuals are from which they originate. 

 All sex-differentiations only serve to bring together the two nuclei essential 

 to the sexual process." 



The opinions of these two authorities are certainly representative, and 

 they both agree in emphasising that the nuclei are all-important, and that it 

 does not matter much about the union of cell-substance. Some objections 

 to this view must be noticed, {a) It is permissible to doubt whether the 



