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 M'ell as that which 

 accompanies the migrating nuclei, 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 laj'^er, 

 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 noevia, 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 {nl.), in which the blastema {bV.) is seen 



* It had been mentioned by earlier writers, Rathke ('37), ClaparMe ('62), and 

 Emerton ('72), but they confounded it with the blastoderm. 



