110 STUDIES IN ELECTRO-PHYSIOLOGY: 



other under the control by the lines of force from the 

 centrosomes, and, being in multiples of two, must divide 

 in equal numbers at the equator. So soon, however, as the 

 two sets of chromosomes regain insulation they again 

 become oppositely electrified, are attracted by the centro- 

 somes, and form two equal groups. 



SEGMENTATION OF THE OVUM. 



Usually, it is said, the two daughter cells are of the 

 same size, but this is not so in the case of the ovum, which, 

 before fertilisation, divides twice (by hetero- and homo- 

 typical mitosis respectively) " into two very unequal 

 parts, the larger of which retains the designation of ovum, 

 while the two small parts which become detached from it 

 are known as the polar bodies. Further, in the formation 

 of the second polar body a reduction-division occurs, and 

 the nucleus of the ovum, after the polar bodies are ex- 

 tended, contains only one-half the number of chromosomes 

 that it had previously e.g., twelve in place of the normal 

 twenty-four in man, and two instead of four in Ascaris 

 Megalocephala (var. bivalvens). Should fertilisation super- 

 vene, the chromosomes which are lacking are supplied by 

 the male element (sperm-cell), the nucleus of which has 

 also undergone, in the final cell-division by which it was 

 produced, the process of reduction in the number of 

 chromosomes to one-half the normal number. The two 

 reduced nuclei which are formed respectively from the 

 remainder of the nucleus of the ovum after extrusion of the 

 polar bodies, and from the head of the spermatozoon, 

 which contains the nucleus of the sperm-cell are known 

 (within the ovum) as the sperm and germ nuclei, or the 

 male and female pronuclei. When these blend, the ovum 

 again contains a nucleus with the number of chromosomes 

 normal to the species." (Schafer.) 



It will thus be seen that while the process of division 



