LECTURE X 

 HEREDITY 



* 



i. THE HEREDITARY EFFECTS OF THE SPERMATOZOON AND EGG 



IN addition to the developmental effects, the spermatozoon has a 

 hereditary effect, inasmuch as it transmits the paternal qualities to the 

 offspring. The experiments on artificial parthenogenesis or chemical 

 fertilization suggest the possibility that the developmental and the heredi- 

 tary agencies in the spermatozoon are connected with different substances. 

 O. Hertwig twenty years ago denned the process of fertilization as the 

 fusion of two nuclei; namely, the egg nucleus and the sperm nucleus. 

 While this fusion is apparently of importance for the hereditary effects, 

 one fails to see how a fusion of two nuclei must cause an egg to develop. 

 The experiments on artificial parthenogenesis indicate clearly enough 

 that the development of the egg can be caused without even the presence 

 of a sperm nucleus. On the other hand, the experiments on merogony 

 show that a fragment of egg protoplasm which has no nucleus can 

 develop when fertilized by a spermatozoon. Delage made extensive 

 experiments in which he cut pieces of protoplasm from the egg of 

 Echinoderms, Annelids, and Mollusks.* These pieces developed when 

 a spermatozoon entered into them. In this case fertilization occurred 

 without a fusion of nuclei, as there was no egg nucleus present. 



It is a very striking fact that for the first stages of development the 

 hereditary influences of the spermatozoon and the egg are by no means 

 equal. It seems that for these first stages the influence of the egg by 

 far exceeds that of the spermatozoon. It may almost be said that 

 the first stages of the embryo are exclusively or almost exclusively 

 determined by the egg, and not by the spermatozoon. This is best 

 illustrated if we hybridize forms whose first stages of development differ 

 radically from each other, e.g. sea urchin and starfish. The pure larvae 

 of both forms go through a blastula and gastrula stage, but then their 

 development becomes strikingly different, inasmuch as the sea urchin 

 larva develops into a pluteus with a skeleton, while the starfish larva 



* Delage, Archives de Zoologie experimentale, Vol. 7, p. 383, 1899. 



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