MUSEUM OF COMPARATIVE ZOOLOGY. 69 
positions directly over the yolk corpuscles. Each cell embraces a large, 
clear, oval nucleus, which is surrounded by an irregularly radiating mass 
of protoplasm. 
The influence of these nuclei upon the protoplasm of the blastema 
soon makes itself evident; a period of rearrangement supervenes in 
which the boundaries of the polygonal areas described above are gradu- 
ally effaced, and the protoplasm of the blastema, as well as that which 
accompanies the migrating nuclet, is grouped into new masses with the 
nuclei as centres. The cells formed in this manner are at first large, 
irregular, and very unequal in size (Fig. 26), but by repeated divisions 
they become smaller, polygonal, and of more nearly uniform dimensions 
(Fig. 27). They ultimately form a continuous layer —the blastoderm 
—in the production of which the whole of the protoplasm of the blas- 
tema has been employed. 
I now turn to a consideration of the internal changes which accom- 
pany the external features already described. 
The structural and other peculiarities of the blastema in the eggs of 
spiders have been subjects of considerable discussion, and therefore 
deserve especial attention. 
Balbiani (’73) was the first to carefully study this layer,* and to 
describe its division into areas. 
Ludwig (76) denied its existence, and located the polygonal areas 
described by Balbiani on the outside of the chorion, they being due, in 
his opinion, to a peculiar arrangement of the granules covering the outer 
surface of that membrane. 
Barrois (78) admitted the existence of the blastema as a partial layer, 
but denied its division into areas; the latter, according to his view, are 
due to intersecting lines of granules located between the chorion and the 
vitelline membrane. 
Sabatier (81) agrees substantially with Balbiani. 
Thus the four observers who have discussed this topic have given 
three irreconcilable explanations of the polygonal areas that Balbiani 
referred to the peripheral layer of protoplasm. 
Sections of eggs during this period afford decisive evidence on the 
points under consideration. In the eggs of Agelena nevia, at least, 
there can be doubt neither as to the existence of this layer, nor as to its 
division into areas. Figure 28 is from a section of an egg containing 
the first segmentation-nucleus (n/.), in which the blastema (0//.) is seen 
* It had been mentioned by earlier writers, Rathke (’37), Claparéde (’62), and 
Emerton (’72), but they confounded it with the blastoderm. 
