CHAPTER XII 

 FERTILIZATION 



We have already pointed out that reduction and fertilization con- 

 stitute the two principal cytological crises in the life cycles of all organisms 

 reproducing sexually. Although the first of these processes was not dis- 

 covered until 1883, some of the grosser features of fertilization had 

 been made out many years previously (Chapter I). But the central 

 feature of this process the union of the two parental nuclei was not 

 known until 1875, when O. Hertwig discovered it in animals, Strasburger's 

 parallel discoveries in plants following in 1877 (Spirogyrd) and 1884 

 (angiosperms). As the finer details of fertilization and the significance 

 of its results become better understood, the aptness of Huxley's (1878) 

 often quoted simile, in which he compares the organism to "a web of 

 which the warp is derived from the female and the woof from the male," 

 becomes increasingly striking. 



We shall first describe the morphology of the fertilization process 

 as it is typically shown in many animals, after which attention will be 

 given to some of its physiological aspects. The second half of the chapter 

 will be devoted to a review of fertilization in the various groups of the 

 plant kingdom. 



FERTILIZATION IN ANIMALS 1 



The Gametes. The spermatozoa of different animals exhibit a 

 surprising variety of form and structure (Fig. 103). What may be 

 referred to as the "typical" spermatozoon consists of three fairly distinct 

 parts: head, middle piece, and tail or fiagellum (Fig. 104). The head 

 represents the nuclear portion of the sperm cell : it consists almost wholly 

 of an extremely compact mass of chromatin. It has an envelope of 

 cytoplasm which in few forms is very conspicuous and in many cases is 

 scarcely distinguishable. Anterior to the nucleus there may be an 

 acrosome, and the end of the head often has the form of a sharp point, 

 the perferatorium. Posterior to the head is the middle piece; this is 

 made up of cytoplasm in which are located the centrosomal structures, 

 together with chondriosomes and other inclusions, such as the "Golgi 

 bodies." The flagellum, or tail, consists of a slender axial filament, 



1 In the preparation of this portion of the chapter the author has drawn very freely 

 upon Professor F. R. Lillie's admirable and concise presentation of the subject, 

 Problems of Fertilization (1919). 



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