132 MR J. T. CUNNINGHAM ON THE 



ovum is exposed to the milt, so that the spermatozoon remains within the 

 protoplasm while the polar globule or globules are being expelled. 



To take up now the first of my two questions. The changes which occur in 

 the blastodisc after the ovum has been exposed to the influence of milt are 

 shown in the figures of the ova of PI. limanda and PL cynoglossus. The separa- 

 tion of the vitelline membrane from the blastodisc occurs almost immediately 

 after the ovum has been placed in sea water containing milt. The protoplasm 

 aggregates at the micropylar pole, and half an hour after fertilisation it projects at 

 the pole considerably into the yolk (PI. III. figs. 3 and 8). At this stage, in ova 

 treated with acetic acid and methyl green, I was able with a high power to see 

 distinctly the male and female pronuclei in close proximity to one another (PI. 

 IV. fig. 2). I was not able to discover the spindle produced from the union of 

 these two bodies. The first segmentation of the blastodisc takes place gradually 

 by the aggregation of the protoplasm round two centres, as in PI. III. fig. 5, and 

 PI. IV. fig. 1. The protoplasm towards each side of the blastodisc projects 

 downwards into the yolk, so that there are now two of these projections 

 instead of one, with a deep broad furrow in the under surface of the blastodisc 

 between them. No furrow on the upper surface of the blastodisc is at first 

 visible. After treatment of the ovum with acetic acid and methyl green at this 

 stage, a nucleus can be made out very distinctly in the two halves of the 

 blastodisc, and these are the only two nuclei in the ovum. The line joining the 

 nuclei is a chord of the sphere of the ovum, and not a radius, as stated by Hoff- 

 mann. The nuclei are best seen when the ovum is placed on the slide with the 

 blastodisc downwards, so that the blastoderm is seen through the transparent 

 yolk, the stained ova being mounted in glycerine. It was from an ovum in these 

 conditions that PI. IV. fig. 3, was taken, the ovum having been killed with 

 acetic acid half an hour after fertilisation. The outline of the blastodisc on the 

 surface of the ovum is not circular, but elliptical, and the plane of division 

 passes through the short axis of the ellipse. This plane of division contains 

 the principal axis of the ovum, by which I mean the axis passing through the 

 centre of the blastodisc and the centre of the ovum. Hoffmann states that the 

 plane of the first division is perpendicular to the principal axis of the ovum 

 (he. cit., p. 105). It seems to me possible that Hoffmann may have been led 

 into this error by the relative positions in which the two nuclei are seen when 

 the ovum is in a certain position with respect to the axis of the microscope. To 

 explain this I must refer to the diagrams shown in figs. 1 and 2. Fig. 1 

 represents a section of the ovum passing through the principal axis and per- 

 pendicular to the plane of the first division of the blastodisc. Now, if the axis 

 of the microscope occupies, with respect to the ovum, the position x y, and the 

 plane which is in focus, perpendicular of course to that axis, occupies the posi- 

 tion shown by the line a b, then the appearance of the section of the ovum seen 



