57 



renders them favourable objects for the study of 

 Apterygote embryology. AVhat is kuowu of the develop- 

 meut of this iusect is due to the researches of Eyder (37), 

 Wheeler,* Miss Claypole (31) and Folsom (32). I'he 

 following account is mainly derived from the observations 

 of the two latter writers. 



The egg is invested by two clearly distinguishable 

 membranes. Firstly, an outer shell-like coat (ch'. in 

 fig. 71), which is very brittle and can be easily dissected 

 off with the aid of a pair of fine needles and, underlying 

 this, is a thin crenated membrane [memh.) These coats 

 may, perhaps, be homologovis with the chorion and 

 vitelline membrane respectively of other Insecta. During 

 the early stages of development, a third and very delicate 

 membrane closely envelopes the embryo, except where it 

 is interrupted by the dorsal or pre-cephalic organ. In 

 addition to these coats, Claypole distinguishes two other 

 Qgg membranes, and the authoress maintains that all five 

 membranes arise from the egg or the blastoderm. 



The unsegmented ovum (fig. 70) is formed of a large 

 central mass of protoplasm, which gives off outwardly 

 directed strands. These strands ramify as they approach 

 the periphery of the egg, and enclose numerous yolk 

 bodies within their meshes. The germinal vesicle early 

 becomes invisible, and does not again appear until after 

 the extrusion of the polar bodies. The latter do not 

 completely separate from the egg, and are eventually 

 resorbed into it. 



The segmentation is especially noteworthy, it being 

 holoblastic but slightly unequal, and results in a solid 

 morula. After the morula has been formed, a breaking 

 down of the cell-boundaries takes place, and many of i\\e 

 nuclei begin to migrate to the periphery of the egg, 

 * A contribution to Insect Embryology. Journ. MorpJi., viii, 1893. 



