474 Mr Doncaster, On the Early Development 



On the Early Development of the Unfertilized Egg in the 

 Sawfiy, Nematus ribesii. By L. Doncastek, M.A., King's College. 



' [Read 2 May 1904.] 



The work of which a preliminary account is here given was 

 undertaken in order to determine (1) what is the function and 

 fate of the polar bodies in a parthenogenetic insect; and (2) 

 whether the early development is different in nearly allied species 

 which give rise to males or females respectively from virgin eggs. 

 The work of Petrunkewitsch *, and the interpretation of it sug- 

 gested by Castle f, make it possible that the polar bodies are 

 concerned with the determination of sex, at least in the Honey 

 Bee, and no group of animals seems better adapted for testing this 

 hypothesis than the Sawflies, many of which lay eggs which 

 develop equally well whether fertilized or not, but produce 

 different sexes in the two cases. 



The only species which I have yet been able to study to any 

 large extent is Nematus ribesii, in which, as is well known, virgin 

 eggs produce only, or almost exclusively, males, while eggs from 

 impregnated females yield a large percentage of females, with 

 a varying number of males. That this difference is due to 

 differential mortality is not certainly disproved, but the evidence 

 makes it improbable ; since, however, my experiments on this 

 matter are not yet complete, it must be left for the present. 



The eggs are laid in rows, on the veins of the under side 

 of currant or gooseberry leaves, and were preserved in a modifica- 

 tion of Gilson's fluid at short intervals, so that all stages from 

 quite fresh eggs up to those ten hours old were obtained. They 

 were cut in position on the leaf and stained with Heidenhain's 

 Iron Haematoxylin. 



In a nearly fresh egg the nucleus is found embedded in a little 

 patch of protoplasm among the yolk on the dorsal side near the 

 anterior end of the egg. This patch of protoplasm I propose to 

 call for brevity the " polar protoplasm." In a few minutes the 

 nucleus begins to divide, forming a spindle perpendicular to the 

 edge of the egg, and in the anaphase it is seen that the spindle 

 fibres are much thickened about the middle of their length. 

 When the chromosomes have reached the poles they do not form 

 nuclei, but two new spindles immediately form, lying in the same 

 line as the previous one. Frontal sections of this stage show that 

 the number of chromosomes after reduction is either seven or 

 eight ; some sections suggest one number, others the other, a fact 



* Petrunkewitsch, Zool. Jahrlmcher Anat. und Ontog. xiv. 1901, and xvn. 1903. 

 + Castle, Bull. Zool. Mus., Harvard, 1903, Vol. xl. 



