SENSORY ORGANS. 



169 



Even in early stages, the outer and anterior part can be distinguished 

 as the rudiment of the eye (Fig. 80 A, p. 157, and Fig. 83, o, p. 163), 

 and the inner, posterior part as that of the optic ganglion (Figs. 

 80, go, and 83, o', o"). The latter is thus, from the very first, in 

 close contact, not only with the rudiment of the eye, but, proximally, 

 with that of the supraoesophageal ganglion. 



The rudiment of the eye is thus only a part of the ectoderm 

 (Fig. 86 A) which becomes multilaminar and produces from its 

 superficial layers the cells of the cornea and crystalline cones, while 

 the lower layers give rise to the retinulae and the pigment-cells. A 

 basal membrane 



secreted on the ,, ft' 



inner surface of 

 this hypodermis 

 (Fig. 86, mb) yields 

 the membrana limi- 

 tans which bounds 

 the eye on the side 

 of the optic gan- 

 glion. Mesoderm- 

 elements are de- 

 posited on this 

 membrane from 

 within, and these, 

 in Mysis, yield 



the pigment of the innermost of the three pigment-layers. In 

 Mysis, according to Nusbaum (No. 39), differentiation occurs at the 

 time when the stalked eye rises from its substratum, the first differ- 

 entiation of the ommatidia appearing in that dorsally curved lamella 

 which, by its curvature, causes the eye to separate from the food-yolk. 

 In this lamella, a very regular arrangement of the cells both into 

 horizontal layers and into vertical columns takes place early. The 

 horizontal lamination separates the corneal cells from those of the 

 crystalline cones, etc. The vertical arrangement gives rise to two 

 alternating columns of cells, which we may distinguish as ommateal 

 and intermediate columns. In each ommateal column, the most 

 superficial transverse layer is formed by two corneal cells (Claus, 

 No. 78) destined to secrete the corneal lens ; below this comes the 

 layer of the crystalline cone, also consisting of two cells. In accord- 

 ance with this number of cells, Grenacher was able to prove the 

 origin of the crystalline cone from two separate segments, the 



Fig. 85.— Left eye of a young Branchipus, seen from the ventral 

 side (after Claus). go, optic ganglion ; k', budding zone for 

 the optic ganglion ; 7c", budding zone for the ommatidia ; m, 

 muscle of the eye. 



