th( 



PHYLUM ARTHROPODA 



599 



seg 



blast 



e earth, or laid in the interior of certain plants or even of 

 animals. The deposition of the eggs, by means of ovipositors, in 

 the leaves or other parts of plants gives rise to swellings the 

 so-called galls, in the interior of which the young Insects live. 

 In the case of many ID sects the eggs are enclosed in a cocoon ; in 

 others they are enclosed in 

 gelatinous or waxy material. 

 The eggs are, for the most 

 part, of relatively considerable 

 size. In form they vary, but 

 the long oval prevails in most 

 instances. The ripe egg is 

 enclosed in two egg-mem- 

 branes an inner, the vitel- 

 line membrane, produced by 

 the egg itself, and an outer, 

 the ckorion, formed from the 

 follicle cells. The chorion, 

 which usually exhibits a more 

 or less elaborate pattern, has 

 one or more apertures or 

 micropyles for the entry of 

 the sperm. The contents are 

 distinguishable into two layers 

 a superficial, consisting of 

 protoplasm, and a central, of 

 nutrient yolk. 



Development. The seg- 

 mentation is usually of a type 

 already referred to (p. 551) 

 as very common among the 

 Crustacea, viz., the superfi- 

 cial segmentation. The actual 

 segmentation (Fig. 489) has 

 only been observed in the 

 case of certain Insects with 

 very little yolk; but there 

 can be very little doubt that 

 Jn ordinary forms with abund- 

 ant yolk the process is in 

 essence the same. The seg- 

 mentation-nucleus, originally situated near the middle of the 

 ovum, divides into a number of nuclei, and most of these migrate 

 towards the surface, and arrange themselves in the form of a 

 sphere almost parallel with the latter ; eventually they reach the 

 surface and coalesce with the peripheral protoplasm, which then 

 becomes divided into cell-areas corresponding with the nuclei. 



"blast 



yk.c 



FIG. 489. A D, successive stages in the seg- 

 mentation of the ovum of an Insect ; blast. 

 blastoderm ; peri, peripheral protoplasm ; 

 seg. segmentation cells ; yk. yolk ; ykc. yolk- 

 cells. (From Korschelt and Heider, after 

 Blochmann.) 





