632 ZOOLOGY SECT. 



ejaculatory duct. Accessory glands, opening into the vas deferens 

 or the ejaculatory duct, secrete cementing material for uniting the 

 sperms into masses, the spermatophores. In most instances the 

 eggs are laid shortly after their fertilisation, only a comparatively 

 few forms, such as the Aphides or Plant-lice, many Diptera, and 

 some Coleoptera, being viviparous. Some Insects, such as the 

 Aphides and the Bees and Wasps, as well as some Lepidoptera 

 and Neuroptera, present us with the unusual phenomenon of 

 parthenogenesis ; i.e., ova are formed, as in ordinary female insects, 

 in organs corresponding to the ovaries of the latter, and are developed 

 without fertilisation. In the case of the Aphides, an autumn 

 generation of completely-developed males and females is followed 

 by a spring generation consisting entirely of females ; these are 

 both parthenogenetic and viviparous. In the Bees, the workers 

 (imperfectly developed females) occasionally produce ova which, 

 without fertilisation, develop into drones (males). In a few species 

 of one or two groups, including the Scale-Insects (Coccidce) and 

 Gall-Insects (GynipidcB), males are never developed, so that repro- 

 duction is exclusively parthenogenetic. Pcedogenesis accompanies 

 parthenogenesis in certain Diptera ; i.e., the larvce or pupce produce 

 ova and embryos without impregnation. 



The eggs when laid are protected from injury by a number of 

 methods ; they may be firmly fixed to the substratum, buried in 

 the 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 Insects the eggs are enclosed in a cocoon ; in others they are 

 surrounded by 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-membranes an inner, the vitelline membrane, produced 

 by the egg itself, and an outer, the chorion, 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 segmentation is usually of a type already 

 referred to (p. 581) as very common among the Crustacea, viz., 

 superficial segmentation. The actual segmentation (Fig. 537) 

 has chiefly been observed in the case of certain Insects with very 

 little yolk ; but there can be very little doubt that in ordinary 

 forms with abundant yolk the process is in essence the same. The 

 segmentation-nucleus, originally situated near the middle of the 

 ovum, divides into a number of nuclei, and most or all of these 

 migrate towards the surface, and arrange themselves in the form 



