No. I.] DEVELOPMENT OF THE COMPOUND EYE. 53 



cle and in both the light traverses these organs in exactly the 

 same direction. Further, as Patten has shown, there is a simi- 

 larity in the nerve-supply. In the vertebrate the optic nerve 

 enters the eye in a mass, passes between the retinal elements 

 and the lens, and gives off its fibres to the distal ends of these 

 elements. In the Crustacea, on the other hand, as shown by 

 Patten for various genera, the nerve to each ommatidium enters 

 the optical portions of the eye separately, but ultimately be- 

 comes distributed to the distal portions of the organ. In the 

 anterior median eyes of the spiders, on the other hand (I am 

 informed by my friend. Dr. E. L. Mark), the optic nerves enter 

 the eye in a cord much as in vertebrates, and are thence distrib- 

 uted to the distal ends of the ommatidium. 



Still these similarities, interesting as they are, do not prove 

 homologies, though I am not yet ready to discuss this point in 

 all its bearings. One prominent difference must, however, be 

 noted. In the arthropod eye the rods and cones are turned 

 toward, in the vertebrate eye away from, the lens. 



The development of the compound eye, as here given, must 

 be regarded as having great weight in settling the question 

 whether the compound eye has arisen by a concrescence of ocelli 

 in the negative. Both ocelli (Locy) and compound eye arise 

 by a single invagination. Were the other alternative true, we 

 should expect the compound eye to have an invagination for 

 each ocellus composing it. 



The observations as yet recorded are not sufficient to throw 

 any great light upon the phylogeny of the arthropod eye, still 

 one or two points may be spoken of The mere fact of invagi- 

 nation must be regarded as indicating an ancestral condition ; but 

 what this condition was is uncertain. The pit or groove must 

 have had sensory functions, and either wall must, for a time, 

 have been like its fellow, as is shown by its having similar 

 nuclei, and by the similar development of rows of nuclei. The 

 position of the eye, too, at the extreme end of the nervous cords 

 would indicate that it was differentiated as a part of the primitive 

 nervous system ; but whether the invagination was originally 

 confined to the eye alone, or whether it is the remnant of a con- 

 dition which formerly extended throughout the whole length of 

 the cords, is another problem. It may be well, however, to call 

 attention to the fact that in all arthropods that have been inve?- 



