422 NATURAL SCIENCE. June. 



Further, the persistent part of the spermatozoon is practically nothing 

 more than a mass of chromatin and a centrosome. The latter is, 

 without doubt, merely an organ for division, although one author 

 has recently tried to show that the centrosome is the bearer of 

 parental qualities to the entire exclusion of the chromatin substance. 

 If it be granted, therefore, that the chromatin is the bearer of the 

 hereditary tendencies, it is of the greatest importance to trace out the 

 changes which the chromatin of the egg- and sperm-nuclei undergo 

 preparatory to copulation. 



Mention has already been made of the "reducing division " of 

 the nuclear matter of the egg-cell by means of the polar bodies. These 

 structures have been noticed for some considerable time, and there 

 has been much speculation as to their real nature. The following are 

 some of the more important suggestions which have been made in 

 order to explain their presence : — 



1. The earlier authors considered them to be excretory products 

 which the ovum had to get rid of. 



2. Kolliker regarded them as a means of rendering the egg-nucleus 

 the same size as the sperm-nucleus. 



3. Flemming thought they represented a parthenogenetic mode 

 of development possessed by the ova of earlier ancestors. 



4. Hertwig and Boveri look upon the formation of the polar bodies 

 as nothing more than unequal cell-division, strictly comparable to 

 what happens in the formation of the spermatozoon. Boveri has 

 constructed two diagrams, reproduced here, to illustrate this point (3), 

 and has invented a new nomenclature for the germ-cells in the 

 various stages of their development. From an inspection of these 

 diagrams it will be seen that the first polar body corresponds to a 

 sperm-mother-cell, while the second corresponds to a sperma- 

 tozoon. While the diagrams illustrate strictly the number of 

 divisions which the ovocyte I. and spermatocyte I. undergo, viz., two, 

 there are more series of cell-divisions than represented in the upper 

 part of the diagram for the zone of germination. These diagrams will 

 be subsequently referred to in considering the fate of the chromosomes, 

 in the reducing division. 



5. Minot and Balfour, as is well-known, regarded the polar bodies 

 as representing outcast male elements, and postulated a somewhat 

 similar process taking place in the formation of the spermatozoon, by 

 which corresponding female elements were got rid of. Thus, these 

 two authors considered the primary germ-cells to be essentially herma- 

 phrodite. 



6. Weismann sees quite a different significance in the polar bodies. 

 According to his earlier view, nuclear matter presents two modifica- 

 tions, viz., germ-plasm and histogenic nucleoplasm. The former is 

 only found in the germ-cell, and is the hereditary substance passed 

 on from parent to offspring. The latter is the element which deter- 

 mines the nature and directs the growth of the cell which contains it. 



