670 NOTES ON THE DEVELOPMENT OF THE ARANEINA. 



cell consists of a normal nucleus, protoplasm, and yolk-spherules. 

 There is a special layer of protoplasm surrounding the nucleus, 

 while the remainder of the protoplasm consists of a reticulum 

 holding together the yolk-spherules. Yolk-cells of this character 

 are seen in Pis. 31 and 32, figs. 10 21. 



The nuclei of the yolk-cells are probably derived by division 

 from the nuclei of the segmentation rosettes (vide Ludwig, No. 5), 

 and it is probable that they take their origin at the time when 

 the superficial layer of protoplasm separates from the yolk- 

 columns below to form the blastoderm. 



The protoplasm of the yolk-cells undergoes rapid division, as 

 is shewn by the fact that there are often two nucleated bodies 

 close together, and sometimes two nuclei in a single mass of 

 protoplasm (fig. 10). It is probable that in some cases the yolk- 

 spheres divide at the same time as the protoplasm belonging to 

 them ; the division of the nucleated bodies is, however, in the 

 main destined to give rise to fresh cells which enter the blasto- 

 derm. 



I have not elucidated to my complete satisfaction the next 

 stage or two in the development of the embryo ; and have not 

 succeeded in completely reconciling the results of my own 

 observations with those of Claparede and Balbiani. In order to 

 shew exactly where my difficulties lie it is necessary briefly to 

 state the results arrived at by the above authors. 



According to Claparede the first differentiation in Pholcus 

 consists in the accumulation of the cells over a small area to 

 form a protuberance, which he calls the primitive cumulus. 

 Owing to its smaller specific gravity the part of the ovum with 

 the cumulus always turns upwards, like the blastodermic pole of 

 a fowl's egg. 



After a short time the cumulus elongates itself on one side, 

 and becomes connected by a streak with a white patch, which 

 appears on the surface of the egg, about 90 from the cumulus. 

 This patch gradually enlarges, and soon covers the whole surface 

 of the ovum except the region where the cumulus is placed. 

 It becomes the ventral plate or germinal streak of the embryo, 

 its extremity adjoining the cumulus is the anal extremity, and 

 its opposite extremity the cephalic one. The cumulus itself is 

 placed in a depression on the dorsal surface of the ovum. 



