TRACHEATA. 



437 



have been studied by Claparede (No. 442), Balbiani (No. 439), 

 Barrois (No. 441) and myself (No. 440), and the close similarity 

 between their embryos leaves but little doubt that there arejio 

 great variations in development within the group. 



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 promi- 

 nences, and occasionally exhibits a pattern corresponding with 

 the areas of the cells which formed it. The segmentation has 

 already been fully described, pp. 1 18 and 119. 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 



fit 



FIG. 198. THREE STAGES IN THE DEVELOPMENT OK CHEUHKK. 



(After Metschnikoff.) 

 pd. pedipalpi ; ab. abdomen ; an.i. anal invagination ; ch. chelicera 1 . 



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, 



