ANIMAL PARTHENOGENESIS 47 



longitudinal split which was visible earlier may still be 

 recognised, and at the next division, which follows rapidly 

 on the first, the transverse halves of the original chromatic 

 loops are divided longitudinally along this split. The essentials 

 of the process are therefore (i) that half the normal number 

 of chromosomes appear in the nucleus ; (2) that they are divided 

 by successive transverse (heterotype) and longitudinal (homo- 

 type) divisions into four cells ; and (3) since the original loops 

 almost certainly consist each of two chromosomes paired end 

 to end, which are separated by the transverse division, each 

 of the four resulting nuclei contains half the normal number 

 of simple chromosomes. 



In animals the reduced cells are normally converted direct 

 into germ-cells ; but in plants a whole generation with reduced 

 nuclei may alternate with that derived from a fertilised egg-cell. 

 In recent years it has been shown 1 that in malignant growths 

 of various sorts nuclear divisions of the " meiotic " type occur, 

 with a reduced number of chromosomes, and it seems not 

 impossible that this fact is connected with the parasitic nature 

 of the growth. The developing germ-cells of animals and 

 frequently the post-meiotic generation in plants behave more 

 or less as parasites having an independent existence at the 

 expense of the rest of the body; and it is therefore perhaps 

 not mere coincidence that similar nuclear structures and modes 

 of division should be found in both gametogenic cells and in 

 malignant growths. 



If this process of chromosome reduction took place in eggs 

 which were not fertilised, and if there were no means of restoring 

 the chromosomes to the normal number during development, 

 it is evident that in every parthenogenetic generation the chro- 

 mosome number would be halved, a process which could not 

 continue very long. Hence it is found that the maturation of 

 parthenogenetic eggs differs, in most cases at least, from those 

 which must be fertilised, and where this difference does not occur 

 some unusual process is found elsewhere in the development 

 of the germ-cells. 



The most frequent type of maturation in parthenogenetic eggs 

 is that only one polar body is formed and there is no reduction 

 in chromosome number. No pairing of chromosomes takes 



1 Farmer, Moore, and Walker, Proc. Roy. Soc, vol. Ixxii. 1903, p. 499 ; 

 also 1904, 1906. 



