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 doI dis- 

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

 been made out many years previously (Chapter I). Bu1 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'e 

 parallel discoveries in plants following in 1877 (Spirogyra) and L884 

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

 of its results become better understood, the aptness of Huxley's 18^8 

 often quoted simile, in which he compares the organism to "a web <»f 

 which the warp is derived from the female and the woof from the mail 

 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 t he 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 flagellum (Fig. KM , The /<<</</ 

 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 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 middl \ this is 



made up of cytoplasm in which are located the centrosomal structun 

 together with chondriosomes and other inclusion-. Buch :i- the Igi 



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



1 In the preparation of this portion of the chapter the author lias drawn very fr» <•!>• 

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

 Problems of Fertilization (1919). 



18 -'73 



