38 BULLETIN OF THE 



stages the basement membrane is strictly ectodermic in origin. In the 

 adult condition it is much thicker than in its early stages, and the 

 greater part of its substance has probably come from the ectoderm, al- 

 though mesodermic cells rest against its proximal face, and possibly may 

 contribute to its formation. 



From the description which I have given, it is evident that the optic 

 disks are thickened regions in the superficial ectoderm, and that these 

 disks are cut by an intercepting membrane into two parts, one deep, 

 the other superficial. The deep part is converted into the optic gan- 

 glion ; the superficial part becomes the retina. So far, then, as the 

 development of the retina in the lobster is concerned, it supports the 

 view that the compound eye in Crustaceans is developed from a simple 

 thickening of the ectoderm. 



In describing the formation of the retina and optic ganglion in the 

 lobster, I have made no mention of an involution. Both Reichenbach 

 and Kingsley have described an infolding in the formation of the eyes, — 

 the former in Astacus, the latter in Crangon, — and it is therefore only 

 natural to look for a similar condition in the eyes of Homarus. Any 

 evidence of an involution in the production of the lobster's eyes is to 

 be sought in the early stages of development. I regret that in the very 

 early stages my material is deficient, and I have not grounds enough to 

 warrant the statement that no involution occurs. All that I can state 

 is, that in all stages which I have examined I have not been able to find 

 any evidence of an involution. The youngest individual which I have 

 studied was one in which the optic disk was about two thirds as thick as 

 that represented in Figure 38. Excepting its thinness and the smaller 

 number of its nuclei, it presented essentially the same appearance as the 

 one which is figured. It will be noticed that near the centre of the 

 disk in Figure 38 there is a space devoid of nuclei. It occurred to me 

 that such a space might represent the last traces of an involution, and I 

 therefore plotted carefully the nuclei in five pairs of disks, some of which 

 were less mature than the disk shown in Figure 38. The result of the 

 plotting was that the light space which is seen in Figure 38 proved to 

 be an individual peculiarity, and I did not find in the arrangement of 

 the nuclei in the other disks any evidence of an involution. 



The plotting of the nuclei, however, brought to light the method by 

 which the disks increased in size. This has already been described, and 

 offers, I believe, an explanation of the fact that in some cases, as for 

 instance in the crayfish, the formation of the eye is attended with an 

 involution, while in other instances, as in Alpheus, no involution is 

 present. 



