108 



EMBRYOLOGY OF INSECTS AND MYRIAFODS 



formed during embryonic life. Among the few accounts we have of their 

 embryonic development are those of Marshall (1935) and of Patten 

 (1887, 1888). 



Of the paurometabolous insects we have Marshall's account of the 

 development of the eye in the female black scale (Saissetia oleae), an 

 example of an extremely simple form. 



The eye rudiment in this insect is a small disk-like area which is 

 formed by a few epidermal cells, slightly larger than those adjacent, on 

 each side of the head of the young embryo. This eye disk invaginates 

 rapidly (Fig. 50^); the margin around the mouth of the invagination 



Fig. 50. — Saissetia oleae. A, invagination of eye rudiment. B, eye vesicle after 

 completed invagination, before development of lens. C, transverse section of eye. (ep) 

 Epidermis, (l) Lens, (rt) Retina, (ves) Vesicle, {vit) Vitreous body. {From Marshall.) 



comes close together until, its cells meeting, a vesicle is finally cut off 

 from the rest of the epidermis in the form of a somewhat spheroidal body 

 lying between the ectodermal surface and the lateral margin of the brain 

 (Fig. 50B). The cuticular layer that covers the embryo external to the 

 invaginated vesicle first becomes smooth and convex, loses its small 

 folds, then forms a bulging disk. At first there are epidermal cells lying 

 beneath the lens, but later they move away from the center toward the 

 margins. The cells forming the vesicle consist of two groups: the outer 

 smaller group of about four cells which form the corneagen cells and the 

 vitreous body; and the inner larger group which gives rise to the retina. 

 The vitreous body then secretes a lens which during embryonic life 

 becomes biconvex (Fig. 50C). Pigment granules form in the retinal 

 cells, which are yellow at first but finally become dark brown or black. 

 It is doubtful if any rhabdoms occur in the ocelli of this insect. No 

 change occurs in the structure of the ocellus during the first instar. At 



