MUSEUM OF COMPARATIVE ZOOLOGY. 43 



internal and an external one. The retinal elements are differentiated 

 earliest in the internal lamella. As the eye becomes more distinctly sep- 

 arated from the body, the internal and external lamellae are unfolded so 

 that they are no longer distinguishable as separate parts. The retina is 

 developed from the thickened layer of hypodermis. So far as Nus- 

 baum's observations extended, the ganglion is produced without an 

 involution. 



The development of the compound eye in Alpheus has been stud- 

 ied by Herrick ('86, '88, and '89). The course of development is 

 almost identical with that of the lobster. 1 The optic disks after they 

 thicken are cut by a basement membrane into a ganglionic and a reti- 

 nal portion. There is no involution connected with the formation of 

 the eye. 



In the introduction to the development of the lobster's eye, mention 

 was made of four structural types which the work of different investi- 

 gators indicated as possible plans for the compound eyes of Crustacea. 

 I have given reasons for excluding three of these. The type of eye 

 which Patten has advocated is unsupported by the embryology of the 

 Crustacea. Eeichenbach and Kingsley misinterpreted structures in the 

 eyes which they studied, and were consequently led to erroneous con- 

 clusions. If the interpretations which I put on the work of these two 

 investigators be admitted, all studies on the development of the com- 

 pound eyes of Crustacea point to one conclusion, namely, that in these 

 eyes the retina originates as a thickened layer of hypodermis, and is not 

 modified by any form of involution. The involution when present is 

 connected with the formation of the optic ganglion only. In the pro- 

 duction of the ganglion, the involution can be replaced by a proliferation 

 of the cells. 



The Optic Nerve. 



The development of the optic nerve 2 is intimately connected with the 

 formation of the intercepting membrane. Before the formation of this 



1 I have had an opportunity of examining Dr. Herrick's unpublished plates on 

 the development of Alpheus, and Dr. Herrick has kindly looked over my figures 

 of the lobster. The correspondence between the method of growth in the eye of 

 Alpheus and Homarus is certainly very close. The few differences that were 

 noticed were such as might be expected between different species. I take this 

 opportunity of thanking Dr. Herrick for his kindness in extending to me the use of 

 his plates. 



2 The optic tracts of a lobster consist of four principal parts. The first of these 

 is the retina, from which nerve-fibres le'ad to the optic ganglion. These three 



