38 



ARACHN1DA. 



takes place. Locy, with whom Kishinouye agrees in the main, explains these 

 markings hy contractions of the egg after it is laid, drawing the periplasm 

 closer to the yolk. The columns of yolk-granules can be separately recognised 

 at the periphery as prominences, and this causes the polygonal markings on the 

 surface. Some of Bai.biani's numerous figures that bear on this point seem to 

 confirm this view, while others contradict it. In these figures, besides the 

 original division of the periplasm into areas, another and true division is shown, 

 caused by the presence of blastoderm cells. Since the egg is said to contract, 

 there might be a regular folding of the vitelline membrane (in the form of 

 polygonal areas), such as is said to occur in Cetochilus (Grobben), but this 

 possibility seems to be excluded, as Locy mentions a perivitelline fluid which 

 appears when the egg contracts, between its surface and the vitelline membrane. 



1. Cleavage and Formation of Germ-Layers. 



Cleavage may here at first be described as total, but passes later 

 into a superficial form. The central nucleus divides, the two 

 daughter nuclei still lying near the centre of the egg (Fig. 21 ^4). 



Fig. 19. — Three ontogenetic stages of Philodromus limbatus (after H. Ludwig, from 

 Balfour's Text-book). 



Although there is no furrow dividing the egg into two, a complete 

 division is indicated, at first, however, only in the yolk. The yolk- 

 granules become arranged radially one behind another in the form of 

 cylindrical columns (Ludwig, Figs. 19 and 21 ^4). These columns, 

 radiating from the centre, become divided into two groups by the 

 division of the nucleus into two (Fig. 19 B). Between them lies 

 formative yolk. As nuclear division proceeds, the two groups of 

 columns, which Ludwig described as rosettes, again divide, and yield 

 four rosettes (Fig. 19 C), which then divide further into eight, 

 sixteen, and thirty-two rosettes, following the usual course of total 



