110 
J. T. CU^INGHAM. 
3. Fix with alcohol 33 per cent, only, transfer to 70 per 
cent., stain with picrocarmine or borax carmine, mount in 
glycerine or balsam. 
4. Osmic acid, picrocarmine ; mount in picrocarminated 
glycerine. This method does not serve for eggs with complete 
envelopes. 
5. Glacial acetic acid may be used for fixing, but is not 
recommended. 
6. For the oldest eggs the author found at the end of his 
researches that the only method was to place living females in 
weak alcohol. The older eggs with thick membranes go on 
developing for a certain time, till, after some months, the 
alcohol kills them, and then fine preparations, showing the 
pronuclei and the early stages of segmentation, are obtained 
by staining with picrocarmine and mounting in glycerine. 
1. First Period of the Maturation of the 
Ovum. 
This stage includes the development of the ovum in the 
ovary and its changes in its passage down the oviduct, until, 
arrived at the upper part of the uterus, it is ready to receive 
the spermatozoon. The author has given a great deal of 
attention and considerable space in his memoir to the struc- 
ture of the ovum, on account of the interest attached to the 
question — Can the position of the embryo be recognised in the 
ovum even before segmentation ? Some indication has been 
given by recent researches of Van Beneden himself and others 
that this question may be answered in the affirmative ; that in 
the ovum of a bilateral animal a right and left side can be 
found corresponding to the right and left side of the embryo, 
and similarly an anterior and posterior end. The author found 
two years previously that in the ovum of Corella paral- 
lelogramma, a simple Ascidian, from the first phases of 
segmentation, the median plane of the embryo, its anterior 
and posterior ends were fixed. Facts of a similar kind have 
