MODERNBIOLOGY 5 43 



directly conflicting with the master's (Haeckel's) theory. Otherwise this 

 work is in many parts somewhat incomplete; thus, the so-called pro- 

 nucleus in the egg is supposed to be developed out of the germinal spot in 

 the ovarial egg, while the rest of the germinal vesicle is assumed to have 

 disappeared — this being stated to apply to the entire animal kingdom. No 

 polar bodies have been observed, nor has it been possible to prove the pene- 

 tration of the spermatozoa into the egg. On the other hand, what is described 

 and illustrated is how two nuclei in the egg gradually approach one another 

 and coalesce, one of which comes from the extreme part of the egg-cell and 

 is therefore characterized as the nucleus of the male sexual product. As a 

 matter of fact, Biitschli and others had previously observed two nuclei unite 

 in the fertilized egg, but had not utilized their discovery for the purpose of 

 a general interpretation; simultaneously with O. Hertwig, van Beneden had 

 published an account of certain fertilization-phenomena in the egg of the 

 Mammalia and had therein expressed the view that, of the two nuclei which 

 he also had observed in the newly-fertilized egg, one is of male and the 

 other of female origin, but he made this statement under reserve as being 

 only a hypothesis, "which may be accepted or rejected." The principle 

 that fertilization consists in the union of the male and the female nuclei was 

 thus without any doubt first set forth by Oscar Hertwig; he was the first to 

 realize the significance of the phenomena and he therefore deserves all the 

 honour for it. 



Our knowledge of fertilization thus made slow progress, with the col- 

 laboration of different investigators. Fol was the first who actually saw 

 (1879) the spermatozoon penetrate the egg, thereby establishing what 

 O. Hertwig had already concluded, that one single male cell performs the 

 act of fertilization. The latter scientist followed up his studies of fertiliza- 

 tion and gradually succeeded in arriving at a clearer view of the subject; in 

 a work on the fertilization of the worms he gives an account particularly of 

 the expulsion of the polar bodies; these bodies, which have already been 

 described by Sven Loven — and possibly still earlier by the aged Carus — 

 were now found to arise through indirect nuclear division, but were still 

 regarded by Hertwig as one of the more incidental phenomena of fertiliza- 

 tion. It was not until later that he discovered them in his first subject of 

 investigation, the egg of the sea-urchin. The next great step towards a solu- 

 tion of the riddle of fertilization was taken by Flemming, who in 1879 

 established the longitudinal cleavage of the chromosomes in indirect cell- 

 division, which was afterwards confirmed by Retzius and Strasburger. In 

 1887 van Beneden published the results of investigation into the fertiliza- 

 tion of the lumbrical ascarid worm of the horse, Ascaris megalocephala, well 

 known on account of its few but large chromosomes. In this animal he found, 

 and afterwards established in other quarters also, the important fact that 



