362 



ARANEINA. 



The ovum is enclosed in a delicate vitelline membrane, enveloped 

 in its turn by a chorion secreted by the walls of the oviduct. The 

 chorion is covered by numerous rounded prominences, and occasion- 

 ally exhibits a pattern corresponding with the areas of the cells which 

 formed it. The segmentation has already been fully described, pp. 97 

 and 98. At its close there is present an enveloping blastoderm 

 formed of a single layer of large flattened cells. The yolk within is 

 formed of a number of large polygonal segments; each of which is 

 composed of large yolk spherules, and contains a nucleus surrounded 

 by a layer of protoplasm, which is prolonged into stellate processes 

 holding together the yolk spherules. The nucleus, surrounded by 

 the major part of the protoplasm of each yolk cell, appears, as a rule, 

 to be situated not at the centre, but on one side of its yolk segment. 



The further description of the development of Spiders applies 

 more especially to Agelena labyrinthica, the species which formed the 

 subject of my own investigations. 



The first differentiation of the blastoderm consists in the cells 

 of nearly the whole of one hemisphere becoming somewhat more 

 columnar than those of the other hemisphere, and in the cells of a 

 small area near one end of the thickened hemisphere becoming 

 distinctly more columnar than elsewhere, and two layers thick. This 



FIG. 198. THBEE STAGES IN THE DEVELOPMENT OF CHELIFEE. (After Metschnikoff.) 

 pd. pedipalpi; ab. abdomen; an.i. anal invagination ; ch. ckelicerae. 



area forms a protuberance on the surface of the ovum, originally dis- 

 covered by Claparede, and called by him the primitive cumulus. 



