94 BULLETIN OF THE 



bered that at this time the ventral area is composed of a thin layer of 

 ectodermic cells ; these cells offer the least resistance to the movement of 

 the yolk mass, which, therefore, takes a ventrad direction. 



During the period of reversion Agelena presents an interesting resem- 

 blance to certain conditions in the development of Oniscus. But accord- 

 ing to Bobretzky ('74, Fig. 15, hd.) the proctodeeum in Oniscus arises 

 some time before the formation of a tail-fold (1. c, Fig. 17, rf.), which, 

 moreover, is never conspicuously indicated. In Agelena the tail-lobe is 

 very prominent, and the fold which results in its formation appears long 

 before the proctodseal invagination. The appearance of this tail-fold, as 

 seen in sagittal sections, is so like that presented by the first stage in the 

 formation of the proctodceum in Oniscus (1. c, Fig. 15) as to suggest the 

 possibility that the infolding in the latter case is really a tail-fold and 

 not the proctodeeum, in which event hd. of Fig. 15 would correspond to 

 rf. of Fig. 16, and the proctodaeum in the latter figure would be a new 

 invagination. The principal objection to this view, aside from the 

 author's reputation for accurate observation, lies in the closeness of the 

 stages of Figs. 15 and 17, which would not seem to allow time for such 

 radical changes. Another and perhaps sufficient objection is, that the 

 invagination in question {Jul. Fig. 15) is lined with columnar epithelium 

 like the proctodteum of the succeeding stage, and that the slight tail-fold 

 is lined with flat cells. But whatever may be the truth with regard to 

 Oniscus, I am certain that in Agelena the tip of what I have called the 

 tail-lobe becomes the morphological end of the body, and that the proc- 

 todseum pierces the tip of this lobe after the reversion of the embryo is 

 nearly completed, and the tail-lobe has become much shortened. 



One fundamental difference supposed to exist between the eyes of 

 Arthropods and those of A^ertebrates, relates to the direction in which the 

 light traverses the retinal elements. In the vertebrate eye the light 

 passes through the cells from their deep to their outer (genetically con- 

 sidered) ends. In the arthropod eye the light was supposed to have the 

 reverse direction ; but that this difference does not exist in the eyes of 

 Agelena is rendered apparent from its manner of development already 

 described. If the proliferation of cells which precedes the invagination 

 led directly to the formation of the eyes, the light would then traverse 

 the percipient elements from their outer to their deep ends ; and it is 

 probable that an ancestral eye of this kind prevailed. In the process of 

 invagination, however, this thickened portion — from which are formed 

 the retinal elements — is completely inverted, and as a consequence the 



