12 



INVEETEBEATA 



CHAP. 



of the egg results ; in most cases this takes the form of simultaneous 

 division into four equal parts. 



In the case of large eggs, like those of birds, it appears that 

 normally a considerable number of spermatozoa enter the egg. One 

 only unites with the female pronucleus and forms the zygote nucleus, 

 the rest divide independently and form groups of small cells which 

 are produced by the aggregation of the cytoplasm round the products 

 of their division. Soon, however, the cells formed round the 

 daughters of the zygote nucleus crush out and kill these other cells, 

 and the former alone enter into the formation of the embryo. 



It appears therefore that the alteration of the surface of the egg 

 so as to exclude supernumerary spermatozoa, which is so marked 

 a feature in small eggs, must be due to some chemical influence 

 radiating from the zygote nucleus, and that in large yolky eggs it 

 does not reach sufficiently far to prevent the entry of extra 

 spermatozoa at some distance from the first one. 



The sequence of events worked out by Agar for the maturation of 

 the nuclei in the male cells of Lepidosiren agrees fairly closely with 

 that described by other workers for other forms. But in many cases, 

 before the first maturation division has taken place, when in the 

 paired or bivalent chromosomes the components are beginning again 

 to separate, a longitudinal split appears at right angles to the split 

 separating the components ; this is the anticipation of the final 

 division of each chromosome into longitudinal halves which occurs in 

 the second maturation division. We thus get quadripartite chromo- 

 somes, which are termed tetrads. Further, many workers maintain 



that in the final pairing of chromosomes 

 which takes place just before the first 

 maturation division, there may in some 

 species be an end -to -end junction, 

 metasyndesis, instead of a side-to-side 

 junction or parasyndesis. 



When a substance like chromatin 

 appears in the ripening eggs and 

 spermatozoa, in exactly the same 

 forms, generation after generation, and 

 when the masses of chromatin con- 

 tinually undergo complicated changes 

 of shape in the same order, it is 

 natural to imagine that such a sub- 

 stance must be of great importance in 

 the main function of the germ cells, i.e. 

 transmitting the hereditary qualities of 

 the parents ; as a matter of fact it was on the casting off of the 

 two polar bodies as a preliminary to development, and on the nuclear 

 changes which accompany this phenomena, that Weismann (1886) 

 founded his famous theory of heredity. According to him the nucleus 

 of the egg was supposed to have a portion of its material charged 



FIG. 5. Polar view of the first 

 maturation division of the male 

 germ cells of Alyihis 2)ilwiil-i<s, 

 in order to show tetrads. 

 (After Wilson. ) 



