THE ORGANS OF THE 'OUTER GERM-LAYER. 



479 



showed (fig. 270 i and 2). Pig men t granules are now deposited in 

 the inner epithelial layer, just as in the outer lamella, so that at last 

 the two are no longer distinguishable as separate layers. 



The mesenchymatic layer which envelops the two epithelial 

 lamellae keeps pace with them in their superficial extension. It 

 becomes thickened and furnishes the stroma of the iris with its 

 abundant non-striated muscles and blood-vessels (fig. 270 3 ). In 

 Mammals (fig. 267 x) this is for a time continuous with the 

 tunica vasculosa lentis (tv), in consequence of which the pupil in 

 embryos is closed by a thin 

 vascular connective - tissue 

 membrane, as has already 

 been stated. 



The part of the optic cup 

 which is adjacent to the pig- 

 ment layer of the iris and ib 

 surrounds the equator of the 

 lens, and which likewise be- aJ 

 longs to the attenuated mar- ''*' 

 ginal zone of the cup (fig. 

 270 cJc], undergoes an inter- 

 esting alteration. In con- 

 junction with the neighboring 

 layer of connective substance, 

 it is converted into the ciliary 

 body of the eye. This process 

 begins in the Chick on the 



ninth or tenth day of incubation (KESSLER), in Man at the end of the 

 second or beginning of the third month (KOLLIKER). The attenuated 

 epithelial double lamella of the cup, in consequence of an especially 

 vigorous growth in area, is laid into numerous, [nearly] parallel 

 short folds, which are arranged radially around the equator of the 

 lens. As in the iris, so here, the adjacent mesenchymatic layer 

 participates in the growth and penetrates between the folds in the 

 form of fine processes. A cross section through the folded part of 

 the optic cup of a Cat embryo 10 cm. long (fig. 271) affords informa- 

 tion concerning the original form of these processes in Mammals. 

 It shows that the individual folds are very thin and enclose within 

 them only a very small amount of embryonic connective tissue (bi ') 

 with fine capillaries, and that, unlike the pigment epithelium of the 

 iris, only the outer of the two epithelial layers (ab) is pigmented, 



Fig. 271. Cross section through the ciliary par 

 of the eye of an embryo Cat 10 cm. long, after 

 KESSLER. 



Three ciliary processes formed by the folding of 

 the optic cup are shown, bi, Connective-tissue 

 part of the ciliary body; ib, inner layer, 

 ab, outer pigmented layer of the optic cup ; 

 bi', sheet of connective tissue that has pene- 

 trated into the epithelial fold. 



