756 TRANSACTIONS OF SECTION K. 



division, as in the cases examined by Weismann, there is no reduction ; and this 

 has been shown to be true also iu Aphis by Miss Stevens (1905) ; but it seems 

 that vrhere two maturation divisions take place in partlienogenetic species there 

 may be either a reduction or two equational divisions in different cases. It is also 

 possible that both methods may occur in different eggs of the same species. 



In 1897 Erlanger and Lauterborn discovered that in the rotifer Asplanchna 

 the parthenogenetic eggs which yield females produce only one polar body, while 

 those giving rise to males produce two. In the bee and some other animals, 

 where there are two maturation divisions, males only are produced from virgin 

 eggs, and Castle (1903) used this fact in formulating his ingenious hypothesis of 

 the segregation of sex-determinaats in a way analogous to the Mendelian segre- 

 gation of inherited charactei-s. But since his paper was published, several excep- 

 tions have been discovered, the most important of which is Aphis, in which 

 parthenogenetic egcr^ yielding both males and females have only one polar body. 

 This has also been found by Morgan in Phylloxera (1906). 



The fate of the polar nuclei iu parthenogenetic eggs seems to vary greatly in 

 different species. In some the polar bodies are thrown out of the egg and lost ; in 

 others the nuclei remain in the &^g, and in that case the first polar nucleus divides 

 at the time when the second is separated from the egg-nucleus. Frequently the 

 two inner polar nuclei, or, in other cases, all three, fuse together and give rise to 

 a group of chromosomes which generally ultimately disintegrate ; but, according to 

 Petrunkewitsch, in the bee the two inner nuclei fuse and give rise to a group of 

 nuclei whicli ultimately form the testi.s of the drone ; and Silvestri in LitomasfAx 

 (a parasitic hymenopteran) says that the polar nuclei proliferate and form a layer 

 which gives rise to the membrane enclosing the embryo. In Artemia Brauer 

 found two types of maturation ; most commonly only one polar body was formed, 

 and the egg-nucleus contained the reduced number of chromosomes, each of which 

 was double. But more rarely two polar nuclei are produced, and in this case the 

 inner sinks back into the egg and coujugates with the egg-nucleus, taking the 

 place of the spermatozoon. An exactly similar process has been described (1904) 

 by Brues in a parasitic insect (Stylopidae). 



The question as to whether the somatic number of chromosomes can be 

 restored if the e^^ begins to develop with the reduced number has given rise to 

 much controversy. In the bee, according to Petrunkewitsch, the reduced number 

 is eight, but he found sixteen in the first mitosis of the egg-nucleus. In the 

 blastoderm divisions, however, he found sixty-four, so that the chromosomes in 

 the maturation divisions mast be regarded as compound, and the increase in 

 number loses some of its signi6cance. So, also, Henking found in Rhodites that 

 the number increases from nine to eighteen ; but he also supposes that in the 

 polar mitoses they are bivalent. In some, at least, of the sawflies the number 

 seems to remain constant, and there is probably no reduction during maturation. 



In the case of eggs made to develop parthenogenetically by artificial treatment 

 there is no better agreement. According to Wilson and Boveri in sea-urchins 

 the chromosomes remain at the reduced number; but Delage states that the 

 number is automatically doubled and the nucleus restored to the somatic con- 

 dition. Kostanecki (1904) has attempted to explain this divergence by obser- 

 vations made on Mactra. When the eggs are caused to develop artificiallj', a 

 spindle is formed, giving rise to two nuclei lying side by side ; but at the next 

 division a single equatorial plate is formed by the two nuclei containing the 

 • somatic number of chromosomes, and this number is maintained in the subsequent 

 divisions. Petrunkewitsch has also investigated artificial parthenogenesis, and 

 finds that the chromosome number varies, owing to splitting sometimes taking 

 place without nuclear division and owing to multipolar mitoses. It appears, 

 therefore, that there is no regularity, and probably in some cases the reduced 

 number is maintained j in others the somatic condition may be restored by 

 abnormal divisions. 



