424 NATURAL SCIENCE. June, 



Thus, the nucleus of a nerve-cell would contain a different kind of 

 histogenic plasm from that of a gland- or muscle-cell. As soon as the 

 ovum has arrived at maturity, its own histogenic plasm has become 

 superfluous, and is therefore got rid of in the shape of polar bodies. 

 When, hov^ever, it became known that, whereas in sexually repro- 

 ducing ova two polar bodies were extruded, in those which developed 

 parthenogenetically there was never more than one,' Weismann was 

 forced to attach another meaning to the second polar body. This he 

 did by suggesting that it was a means of keeping the number of 

 ancestral units in germ-plasm from increasing ; were there not some 

 such method the germ-plasm would be doubled at every fertilisation, 

 and soon grow too large to be contained in the nucleus. It is possible 

 that this idea was based on certain erroneous observations as to the 

 exact method of nuclear division in the formation of the second polar 

 body. The point is far too complex to be entered upon here, but it 

 has been adequately discussed, and criticised by Boveri (3) and others. 

 After the publication of Hertwig's (10) admirable researches on 

 the spermatogenesis of Ascaris, Weismann considerably modified his 

 views. It was shown, or at any rate Weismann has gathered from 

 Hertwig's work, that the youngest sperm-mother-cells possess nuclei 

 which are characterised in the active stage by the presence of four 

 chromosomes. These cells (spermatogones, to use Boveri's nomen- 

 clature) increase by ordinary karyokinetic division, i.e., longitudinal 

 splitting of their chromosomes up to a certain point. When this is 

 reached, the cells cease to multiply, but grow considerably in size, 

 and their nuclei pass into the resting condition. On again becoming 

 active, the nuclei are seen to possess not four rods as heretofore but 

 eight. At this point the reducing division sets in. Each cell (sper- 

 matocyte I.) divides into two (spermatocyte II.), each of which again 

 divides, forming two daughter-cells, which are the true spermatozoa 

 (Spermatids) (5^^ diagram). It must be noticed, however, that in both 

 these latter divisions there is no longitudinal splitting of the chromo- 

 somes, but half the number of rods pass into the daughter-nuclei. 

 Since there are two reducing divisions, it will be seen that each 

 spermatozoon eventually comes to possess two rods,^.^., half the number 

 contained by the original spermatogone. The process in the formation 

 of the ovum is exactly similar to the above, the sole difference being 

 that, whereas in the male cells the reducing division of the sperma- 

 tocyte I. gives rise to four equal spermatozoa, that of the ovocyte I. 

 gives rise to three polar bodies- and the ovum, of which only the last 

 is functionally active. Therefore, the ovum before the extrusion of the 

 polar bodies is the homologue of the sperm-grandmother-cell or 

 spermatocyte I. 



1 Blochmann, however, has described two polar bodies in the parthenogenetic 

 ova of Apis and Lipayts, and Brauer has shown that the same is the case in the 

 parthenogenetic eggs of Branchipus and Artcmia. 



2 The first polar body divides into two, making three in all. 



