MUSEUM OF COMPARATIVE ZOOLOGY. 49 



The substance in which the nuclei lie contains a few granules of pig- 

 ment. Below the proximal end of the cone the nuclei of the proximal 

 part of the retina can be seen. These are not so definitely arranged 

 that they can be counted. They occur on several planes. Fortunately, 

 their cells in this stage have very distinct outlines, and a short distance 

 below the nuclei one can see with perfect clearness the seven retinulae 

 which surround each rhabdome (Fig. 57, rtn) '. px.). Occasionally, as is 

 seen in Figure 57, the nucleus of the retinula occupies a position in its 

 cell as deep as the plane of this section. The proximal retinulae contain 

 a few pigment granules. On account of the great similarity of the 

 groups of proximal retinulae, I have not been able to plot and super- 

 impose sections with certainty ; and as the nuclei of the proximal retin- 

 ulae are placed at different levels I have not succeeded in identifying 

 an eighth nucleus, which, it will be remembered, was pointed out in 

 the histology of the adult eye as probably representing a degenerate 

 retinula. 



At this stage the first trace of the rhabdome appears (Fig. 57, rhb.). 

 It is a cylindrical thickening in the centre of each group of proximal 

 retinulae, and extends from a short distance below the crystalline cone 

 very nearly to the basement membrane. From its earliest appearance 

 it is divided into four segments, which bear the same relation to the sur- 

 rounding retinulae as they do in the adult. (Compare Figs. 58 and 34.) 

 Nothing has been observed in the development of the rhabdome which 

 indicates the significance of the four lines by which the segments of the 

 rhabdome are separated. 



Owing to a lack of distinctness in the tissue near the basement 

 membrane, I have been unable to identify the individual retinulae in 

 that region. What I have observed is, that fibres pass from the retina 

 through the basement membrane and into the optic ganglion. Pre- 

 sumably these fibres are from the proximal ends of the retinulae, and are 

 grouped as I have described in the retina of mature lobsters. 



At this stage the only observation which I have made bearing on the 

 question of nerve termination is as follows. In each proximal retinula 

 near the rhabdome there are one or two fibres which extend nearly the 

 whole length of the retinula. In transverse sections they of course 

 appear as dots (Fig. 58, fbr'.), and might be mistaken for the remains 

 of pigment granules were it not fir* their sharper outlines and the regu- 

 larity of their arrangement. I am of opinion that they are the first indi- 

 cations of nerve-fibrillae, which, as I have pointed out in the section on 

 Histology, lie in the adult eye next the rhabdome. 



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