Procephalic Lobes of Epeira Cinerea. 423 



peculiar structure, located near the uueleus, which he calls the yolk 

 nucleus. 



Segmentation of the egg begins, according to these authors, by the 

 division of the centrally located nucleus into two, four, eight, etc., 

 parts. This affects the whole structure of the egg, the yolk splitting 

 with each division of the nucleus into a corresponding number of 

 parts, each part containing one of the products of nuclear division 

 (Kishinouye, 19). In this manner the yolk becomes separated into 

 the numerous, radially arranged masses already noted, to which the 

 name "yolk columns" has been applied. 



The separation of the yolk into columns discloses a cavity in the 

 central part of the egg. This has been regarded by some investigators 

 as representing the segmentation cavity. It is obliterated at a later 

 period by the incrowding of the yolk masses. 



The nuclei resulting from the repeated division of the segmenta- 

 tion nucleus gradually pass along the lines of protoplasm which radi- 

 ate outward between the yolk columns, and finally appear on the 

 surface of the egg. They are first seen at the points where the radiat- 

 ing strands of protoplasm unite with the periplasm, and are equally 

 distributed over the entire surface of the egg (Morin, 30; Kish- 

 inouye, 19). 



A certain amount of protoplasm accompanies the nuclei in their 

 outward migration (Korschelt and Heider). This unites with the 

 periplasm. In this manner the cellular elements are formed which 

 lay the foundation of the blastoderm. Shortly after this period, by 

 the shifting of the yolk, the nuclei are no longer seen at the points 

 where the protoplasmic radii and the periplasm meet, but lie more 

 or less directly in the areas formed l)y the surfaces of the yolk columns. 

 Fig. 1, nu. 



2. Formation of the Blastodisc. — Following the formation of the 

 blastoderm as a single, uniform layer of cells surrounding the egg, 

 there is a concentration of the nuclei on one surface, namely that on 

 which the embryo arises and Avhich may be regarded as the ventral 

 surface of the egg. This concentration of nuclei has been observed 

 by most investigators ; but there is a variety of opinion as to the 

 manner in which the additional cells originate. 



