294 EMBRYOLOGY 



one. It deepens our knowledge of the process of fertilization because 

 it adds to the previously considered chemical and physical elements 

 the morphological element which is so important in the phenomena 

 of life and inheritance by showing that the essential substance has 

 a definite form. Also the new experiences with regard to the im- 

 portant role which the nucleus plays in cell-division, as well as for 

 the study of fertilization, come into play, and at the same time the 

 formation of the polar bodies as a preparatory stage for the con- 

 jugation of the nuclei is explained in a far better way than was 

 previously possible." 



On the basis of these observations fertilization may be looked upon 

 as union between two different cells which arise from a male and a 

 female individual. The essential in this process is evidently the union 

 or, to use the expression of Weismann, the amphimixis of the egg and 

 sperm-nuclei. That this is a general law of biologic nature is now 

 doubted by none. For fertilization is the same process in all classes of 

 animals as in the Echinodermata. Many of these, such as Ccelenierata 

 and Vermes, as Tunicata and Mollusca, as Crustacea and Insecta, have 

 been investigated by various scientists. The numerous Vertebrata, 

 in which the process has been followed, Mammalia, Reptilia, and 

 Amphibia, Cyclostomen and Amphioxus, all show the same process. 



The discovery of the fertilization process in animals has immedi- 

 ately brought about similar discoveries in the plant kingdom. The 

 fertilization of P keener ogamia, previously studied without result 

 by many investigators, was now quickly explained by Strasburger. 

 Our knowledge in this and other points was completed by Guignard, 

 Nawashin, and others. We now know that the pollen grain, which is 

 analogous to the spermatozoon of animals, carries into the egg-cell 

 of the ovary a sperm-nucleus which combines with the egg-nucleus. 

 The correspondence is even greater in the Cryptogamia, as here ex- 

 ternally the vegetable spermatozoid is very similar to the animal 

 spermatozoon, and fertilization proceeds in a similar way. Even 

 among the lowest organisms, Infusoria, Rhizopoda, Flagellata, Algae, 

 and Fungi, the process of fertilization is seen to be the same. 



By this natural law of sexual generation, which is now so surely 

 founded and based upon a complete series of observations, the old 

 discussion which once played so great a role in the history of the sci- 

 ences and engaged for a long time the naturalists and philosophers, 

 the strife between the Ovists and the Animalculists, has been decided. 



From the sixteenth to the eighteenth century the dogma of pre- 

 formation ruled : the doctrine that the embryo of a being was built 

 up of the same organs and parts as the parent, and thus was nothing 

 less than an extraordinarily minute reproduction of it. Most scien- 

 tists (S\vammerdam,Harvey, Spallazani, Bonnet, Haller) looked upon 

 the egg as the preformed embryo, as may be seen from the well- 



