396 A. J. NICHOLSON 
INTRODUCTION. 
THE examination of mosquito ovaries was first commenced 
with the idea of finding out at what period the ovaries of the 
hibernating females commence to develop, so that an accurate 
knowledge of the time at which the mosquito lays the first 
egos of the season might be determined. From an examination 
of sections of the ovaries, it soon became evident that the 
oocyte nucleus behaved in a somewhat unusual manner during 
the period of yolk formation. I therefore decided to examine 
this in detail and at the same time observe what might be 
termed the grosser anatomy of the developing ovary and 
oocyte. An immense amount of work has been done on the 
oogenesis of insects, but most of this has been confined to the 
detailed examination of the complicated nuclear changes which 
take place during this period. The mosquito, however, is 
peculiarly unsuitable for the study of the differentiation of the 
oocyte and of the prophases of maturation which takes place 
in the end chamber. As this is very small, in order to examine 
some of the stages, it would be necessary to cut very thin 
sections of ovaries containing oocytes with large yolk-masses 
and in some cases chorion as well. This is an operation which 
I found quite impossible to perform. The finer structure of 
the oocyte nucleus has therefore only been studied where it 1s 
rendered necessary in order to give a connected account of the 
development of the oocyte. 
To the best of my knowledge the only references to the 
development of the ovarian egg of the mosquito are contained 
in two short papers by Christophers. In one of these (2) he 
gives a very general description of the ovary and egg-follicles, 
while in the other (8) he describes the development of the 
egs-follicle from the examination of fresh material. As the 
information in both these papers is of a very general nature it 
has been found necessary to repeat portions of it, as otherwise 
a connected account of the development of the ovary could 
not be given. 
I will take the opportunity here of expressmg my deep 
