FERTILIZATION 279 



its group of chromosomes, the two groups arranging themselves on a 

 common spindle which organizes when the nuclear membranes dissolve. 

 The first cleavage mitosis (first embryonal division) then ensues, and 

 the two daughter nuclei receive longitudinal halves of each and every 

 chromosome. Thus in the act of fertilization, in both animals and plants, 

 each parent furnishes the offspring with a haploid set of chromosomes, 

 the two intermingled sets constituting the diploid set of the new individual. 

 Since every chromosome divides equationally at every subsequent somatic 

 mitosis, every cell of the body receives half of its chromosome complement 

 from each parent. The cardinal importance of this fact in connection 

 with current theories of heredity will be apparent in subsequent chapters. 



The two groups of chromoosmes, paternal and maternal, can often be 

 distinguished not only on the spindle of the first cleavage division, but 

 in several divisions thereafter. As examples may be cited Cyclops 

 (Riickert 1895; Hsecker 1895), Crepidula (Conklin 1901), and Crypto- 

 branchus (Smith 1919) (Fig. 109). This phenomenon is especially evident 

 in hybrids (p. 160). There is much reason to believe that the chro- 

 matins of the two parents, although intermingled in the nuclei of the 

 offspring, never actually fuse, unless it is at the time of synapsis in the 

 next maturation; and it has already been pointed out (Chapter XI) that 

 they may not fuse even then. This fact also has an important bearing on 

 the chromosome theory of heredity. 



The Centrosome. (See Wilson 1900, pp. 208 ff.) Shortly after the 

 entrance of the spermatozoon into the egg (Figs. 106-108) an aster devel- 

 ops at the base of the sperm head, and in the aster a centrosome appears. 

 Since the centrosome thus arises in the position of the middle piece, and 

 since the centrosome of the spermatid is included in the middle piece dur- 

 ing spermatogenesis, a widely accepted theory has been that the newly 

 appearing centrosome is in reality that of the spermatid. Whatever its 

 origin, it soon divides to form the two which function in the first cleavage 

 mitosis. These facts had much to do with the formulation of a theory of 

 fertilization set forth by Boveri (1887, 1891), who was much impressed by 

 the conspicuous part played by the centrosomes in cell-division. Accord- 

 ing to Boveri's theory the egg is not able to undergo division because of 

 the lack of any centrosome to initiate the process, while the spermatozoon 

 has a centrosome but not sufficient cytoplasm in which to act. Through 

 the union of the gametes all the organs necessary for division are brought 

 together and cleavage proceeds. This theory has recently been recalled 

 by Walton (1918) in his work on Ascaris canis. 



Another early view of the origin of the cleavage centrosomes was that 

 of van Beneden (1887) and Wheeler (1895, 1897), who believed them to be 

 the centrosomes of the egg cell. 



The theory that the cleavage centrosomes arise from both egg and 

 spermatozoon is of some historic interest. It was suggested by Rabl 



