86 F. M. BALFOUft. 



situated in relation to the embryo ? Before attempting to 

 answer these questions I will shortly describe the develop- 

 ment, so far as I have made it out, for the stages during 

 which the cumulus is visible. 



The first change that I find in the embryo (when examined 

 after it has been hardened)^ is the appearance of a small, 

 whitish spot, which is at first very indistinct. A section 

 through such an ovum (PI. IX, fig. 10) shows that the cells 

 of about one half of the ovum have become more columnar 

 than those of the other half, and that there is a point {pr. c.) 

 near one end of the thickened half where the cells are more 

 columnar, and about two layers or so deep. It appears tome 

 probable that this point is the whitish spot visible in the hard- 

 ened ovum. In a somewhat later stage (PL YIII, fig. 1) the 

 whitish spot becomes more conspicuous (pc), and appears as 

 a distinct prominence, which is, without doubt, the primitive 

 cumulus, and from it there proceeds on one side a whitish 

 streak. The proniinence,as noticed by Chiparede andBalbiani, 

 is situated on the flatter side of the ovum. Sections at this 

 stage show the same features as the previous stage, except 

 that (1) the cells throughout are smaller, (2) those of the 

 thickened hemisphere of the ovum more columnar, and (3) 

 cumulus is formed of several rows of cells, though not 

 divided into distinct layers. In the next stage the appear- 

 ances from the surface are rather more obscure, and in some 

 of my best specimens a coagulum, derived from the fluid 

 surrounding the ovum, covers the most important part of the 

 blastoderm. In PI. VIII, fig. 2, I have attempted to repre- 

 sent, as truly as I could, the appearances presented by the 

 ovum. There is a well-marked Avhitish side of the ovum, 

 near one end of which is a prominence (jyc), which must, 

 no doubt, be identified with the cumulus of the earlier stages. 

 Towards the opposite end, or perhaps rather nearer the 

 centre of the white side of the ovum, is an imperfectly 

 marked triangular white area. There can be no doubt that 

 the line connecting the cumulus with the triangular area is 

 the future long axis of the embryo, and the white area is, 

 without doubt, the procephalic lobe of Balbiaui. 



A section of the ovum at this stage is represented in 

 PL IX, fig. 11. It is not quite certain in what direction 

 the section is taken, but very probably it is somewhat 

 oblique to the long axis. However this may be, the section 



^ I was unfortunately too much engaged, at the time when the ego-s 

 were collected, to study them in the fresh condition ; a fact which has 

 added not a little to my difficulties in elucidating the obscure points in 

 the early stages. 



