ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 291 



Polyspermy sometimes exists normally, but even then all the sperma- 

 tozoa save one are normally absorbed by the egg-cytoplasm. If their 

 centrosomes persist they may form independent centres for cell-division, 

 and interfere — sometimes fatally — with normal development. Abnormal 

 development also follows where the egg-nucleus unites with more than 

 one sperm-nucleus. Thus the mere entrance of a spermatozoon into 

 an egg cannot be regarded as in itself the efficient stimulus to normal 

 development. It is possible, by etherising an egg, to prevent the union 

 of the sperm-nucleus with the egg-nucleus, and then one-half of the 

 egg, with the egg-nucleus, develops parthenogetically, while the other, 

 with the sperm-nucleus, develops merogenetically. Therefore Petrun- 

 kevitch holds to the old definition of fertilisation, and calls an egg 

 fertilised only when the union of the nuclei is accomplished. Apart 

 from fission, there are three kinds of propagation without fertilisation : — 

 (a) budding, through successive regular mitotic divisions proceeding 

 from one or several cells ; (b) parthenogenesis, from an unfertilised egg, 

 introduced by a maturation division ; and (c) paadogenesis, as in 

 Cecidomyia, which may be larval parthenogenesis or nearer budding, 

 according as a process of maturation does or does not occur, which is 

 still a problem. 



In the majority of parthenogenetic eggs — the single exception being 

 that of certain insects — there is but one maturation division and no 

 reduction of chromosomes, the opportunities for variation being thus 

 much restricted. The exception to this rule is found in those insects 

 in which only one sex develops parthenogenetically, and in the male 

 individuals in those cases where both sexes develop parthenogenetically. 

 In such cases two maturation divisions take place with a corresponding 

 reduction in the number of chromosomes. 



Microscopic study and that only is able to show without error 

 whether an egg is fertilised or not. The development of an aster 

 around the oentrosome of the spermatozoon after it has entered the 

 ovum facilitates the finding of the sperm-nucleus, which keeps its place 

 close behind the migrating centrosome. This sperm-aster develops in 

 the same way throughout the entire animal kingdom ; in polyspermic 

 bee-eggs similar asters appear in numbers equalling those of the sperma- 

 tozoa which enter the egg ; in the so-called drone-egg an aster would 

 surely develop should a spermatozoon enter it. In spite of all criticism, 

 the parthenogenesis of the drone-egg remains certain. 



In parthenogenetic eggs the egg-centrosome remains active ; in 

 Artemia sali/ia, at least, we can easily see that it moves alone towards 

 the centre of the egg to await there the nucleus, which, after the single 

 maturation-division, 1 " is destined to become the first cleavage-nucleus. 

 Thus does the egg-centrosome in parthenogenetic eggs maintain its 

 individuality through all cell-generations. May it be, however, that 

 in pure parthenogenesis the ostensibly female centrosomes are descen- 

 dants of a remote ancestral male centrosome ? 



Still more important biological problems are connected with the 

 behaviour of the chromosomes" in parthenogenesis. In pure partheno- 

 genesis they remain unreduced in number ; but in the exceptional cases 

 the number is reduced. But Petrunkevitch has shown that the number 

 of chromosomes in the first cleavage nucleus of the drone-egg again 



x 2 



