FERTILIZATION 279 



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

 common spindle which organize- when the nuclear membranes dissoh 

 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 ferl ilizai ion, in bol h animals and plant 

 each parent furnishes the offspring with a haploid sit of chromosome 

 the two intermingled sets constituting the diploid set of th( m w dual. 



Since every chromosome divides equatio natty at every subsequent somatic 

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

 from each parent. The cardinal importance of this fad 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 

 (Ruckert 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 ! thai 

 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 fT.) 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 appea 

 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 lias been that the newly 

 appearing centrosome is in reality that of the spermatid. \\ 'hat ever its 

 origin, it soon divides to form the two which fund ion in I he firsi cleavi 

 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 Bpermatoaoon 

 has a centrosome but not sufficient cytoplasm in which toact. 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 nun's. 



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

 of van Beneden (1887) and Wheeler (1895, L897 , 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 sug d by Rabl 



