LECTURE IX 



FERTILIZATION 



i. THE SPECIFIC CHARACTER OF THE FERTILIZING POWER OF THE 



SPERMATOZOON 



IT is comparatively easy for the physicist to give to his data the form 

 of a mathematical law, inasmuch as the independent variables are 

 mostly in evidence, and all that remains to be done is to find the formula 

 which expresses the relation between the variable and the function. In 

 biology the independent variable is generally unknown, and the main 

 energy of the investigator must be devoted to discovering this variable. 

 The history of the problem of fertilization is extremely instructive in 

 this regard. Although the fact that many animals, e.g. fishes, birds, 

 etc., develop from an egg has been known as long as man has observed, 

 it was not until 1827 that von Baer discovered the mammalian egg; 

 and although Leuwenhoek, or a pupil of his, discovered the existence 

 of spermatozoa in the sperm as early as 1677, it was not until 1843 that 

 the fact was really established that generally the development of the egg 

 is caused by the entrance of a spermatozoon. 



As far as we know at present the entrance of a spermatozoon 

 into the egg has two kinds of effects which must be kept apart : the first, 

 namely, the starting of the process of development, the developmental 

 effect; the second, the transmission of the paternal qualities to the 

 new organism, the hereditary effect. We shall first discuss the develop- 

 mental effects of the spermatozoon upon the egg, raising the question 

 whether this effect of the spermatozoon is specific or general ; that is to 

 say, whether a spermatozoon can cause only the development of an egg 

 of the same species or of any egg. It is well known that animals belong- 

 ing to the same family, e.g. various kinds of dogs, the horse and the 

 donkey, can be successfully crossed. In fishes, also, it has long been 

 known that various types of hybrids can be easily obtained. Spallan- 

 zani and other observers were never able to obtain hybrid larvae among 

 the Batrachians. Pfliiger, however, found that the first segmentations 

 can be produced in the eggs of Rana jusca by the sperm of a salamander 



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