NOTES ON THE DEVELOPMENT OP THE ARA.NEINA. 171 
shows 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 corresponds 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 
thickened ridge of the blastoderm. In any case, it is clear 
that the white hemisphere of the blastoderm is a thickened 
portion of the blastoderm, and that the thickening is in part 
due to the cells being more columnar, and, in part, to their 
being more than one row deep, though they have not become 
divided into two distinct germinal layers. It is further clear 
that the increase in the number of ceils in the thickened 
part of the blastoderm is, in the main, a result of the mul- 
tiplication of the original single row of cells, while a careful 
examination of my sections proves that it is also partly due 
to cells, derived from the yolk, having been added to the blas- 
toderm. 
In the following stage which I have obtained (which 
cannot be very much older than the previous stage, because 
my specimens of it come from the same batch of eggs), a 
distinct and fairly circumscribed thickening forming the 
ventral surface of the embryo has become established. Though 
its component parts are somewhat indistinct, it appears to 
consist of a procephalic lobe, a less prominent caudal lobe, 
and an intermediate portion divided into about three seg- 
ments; but its constituents cannot be clearly identified with 
the structures visible in the previous stage. I am inclined, 
however, to identify the anterior thickened area of the pre- 
vious stage with the procephalic lobe, and a slight protu- 
berance of the caudal portion (visible from the surface) with 
the primitive cumulus. I have, however, failed to meet 
with any trace of the cumulus in my sections. 
To this stage, which forms the first of the second period 
of the larval history, I shall return, but it is necessary 
now to go back to the observations of Claparede and Bal- 
biani. 
There can, in the first place, be but little doubt that what 
I have called the primitive cumulus in my description is the 
structure so named by Claparede and Balbiani, 
