6/2 NOTES ON THE DEVELOPMENT OF THE ARANETNA. 



spicuous (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 prominence, as noticed by 

 Claparede and Balbiani, is situated on the flatter side of the 

 ovum. Sections at this stage shew the same features as the 

 previous stage, except that (i) the cells throughout are smaller, 

 (2) those of the thickened hemisphere of the ovum more columnar, 

 and (3) the cumulus is formed of several rows of cells, though not 

 divided into distinct layers. In the next stage the appearances 

 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 PL 30, fig. 2, I have attempted to represent, as truly as I 

 could, the appearances presented by the ovum. There is a 

 well-marked whitish side of the ovum, near one end of which is 

 a prominence (/>.), 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 Balbiani. 



A section of the ovum at this stage is represented in PL 31, 

 fig. ii. It is not quite certain in what direction the section is 

 taken, but I think it probable it is somewhat oblique to the long 

 axis. However this may be, the section shews that the whitish 

 hemisphere of the blastoderm is formed of columnar cells, for 

 the most part two or so layers deep, but that there is, not very 

 far from the middle line, a wedge-shaped internal thickening of 

 the blastoderm where the cells are several rows deep. With 

 what part visible in surface view this thickened portion corre- 

 sponds is not clear. To my mind it most probably corresponds 

 to the larger white patch, in which case I have not got a section 

 through, the terminal prominence. In the other sections of the 

 same embryo the wedge-shaped thickening was not so marked, 

 but it, nevertheless, extended through all the sections. It 

 appears to me probable that it constitutes a longitudinal thick- 

 ened ridge of the blastoderm. In any case, it is clear that the 

 white hemisphere of the blastoderm is a thickened portion of the 



