48 SCIENCE PROGRESS 



place beforehand, and the single nuclear division consists in 

 a longitudinal splitting of each chromosome, and a separation of 

 the halves into different nuclei. This is found in parthenogenetic 

 eggs of the Daphniidae 1 and Ostracoda 2 among the Crustacea, 

 and the Aphididae 3 among insects, which include the majority 

 of parthenogenetic species if cases which are commonly classed 

 as budding are excluded. In this group there is no visible 

 difference in the maturation of the eggs, whatever kind of 

 individual those eggs are destined to produce ; males and sexual 

 and parthenogenetic females all seem to have identical maturation 

 processes. 



In the rotifer Asplanchna, on the other hand, 4 it is stated that 

 eggs which will develop into females have only one polar body, 

 but those which yield males have two. A somewhat similar case 

 is that of Hydatina? in which female parthenogenetic eggs begin 

 a polar mitosis which is never completed ; but the male and 

 fertilisable female eggs have one complete polar division. 



But quite a large number of parthenogenetic eggs have 

 two maturation divisions and produce two polar nuclei, or 

 three if the first one divides, as is frequently the case. This 

 is so in all the Hymenoptera which have been examined, viz. 

 the bees, wasps, sawflies, gallflies, and some belonging to the 

 parasitic families. Two polar divisions are also recorded for 

 the stick insect Bacillus. 6 In Artemia 1 (Crustacea) and in one 

 of the Stylopidae 8 (insects) the formation of two polar bodies 

 has been recorded ; but it is stated that the second may return 

 into the egg and conjugate with the egg nucleus, so apparently 

 taking the part of a spermatozoon. In Artemia, however, this 

 is not invariable, for often only one polar division takes place. 



But in the other cases in which two polar divisions take 

 place in a parthenogenetic egg, it is of interest to know whether 

 reduction of chromosomes takes place or not. In some species, 

 e.g. of gallflies and sawflies, the species may be reproduced for 

 many generations without the intervention of a male, and if the 



1 Weismann and Ischikawa, Ber. Nat. Ges. Freiburg, iii. 1887. 



2 Woltereck, Zeit. Wiss. Zoo. lxiv. 1898. 



3 Stevens, Carnegie Inst. No. 51, 1906. 



4 Erlanger and Lauterborn, Zoo. Anz. xx. 1897, p. 452. 



5 Lensen, La Cellule, xiv. 1898, p. 421. 



6 Von Bahr, Zool. Jahrb. Anat. xxiv. p. 175. 



7 Brauer, Arch. Mikr. Anat. vol. xliii. 1893. 



8 Brues, Zool. Jahrb. Anat. xviii. p. 241. 



