DEVELOPMENT OF THE LENS 



53 



of invertebrates. A reference to the central eyes of Euscorpius (Fig. 95, 

 Chap. 11, p. 133) or a section through an ocellus of a Dytiscus larva 

 (Fig. 38) will show, however, that one type of lens formation can pass into 

 the other ; and it is obvious that if the optic vesicle is withdrawn away 

 from the surface ectoderm, as has occurred in the phylogenetic history 

 of the pineal eyes of vertebrates, the cutaneous ectoderm or epidermis 

 can no longer form or participate in the ontogenetic development of the 

 lens of the " pineal eye," although it may have done so primarily. With 

 reference to the ontogenetic development of the " pineal eye," Beard, 

 who studied the pineal organ in the Ammoccete or larval form of Petromyzon, 



Fig. 38. — Vertical Section through Ocellus of Larva of Dytiscus Mar- 



ginalis, across its shortest diameter. (after gljnther.) 



ect. : unaltered ectoderm. s.v.c. : short visual cell. 



l.v.c. : large visual cell. s.v.r. : short visual rod. 



l.v.r. : long visual rod. vitr. : vitreous mass secreted by 

 n.f. : nerve fibres. perineural cells at mouth of pit. 



thought it possible that a portion of the cutaneous ectoderm might be 

 included in that part of the neural tube which gives rise to the pineal 

 organ, these cells being cut off at the time when the neural tube becomes 

 separated from the surface, or cutaneous ectoderm. Whether this explana- 

 tion is valid or not, it is quite certain that phylogenetically the neural 

 ectoderm has been derived from the cutaneous ectoderm ; moreover, 

 ontogenetically the neural ectoderm is continuous with the cutaneous 

 ectoderm in the early stages of embryonic development. Further, there 

 exist not only transitional forms between the types of lens developed from 

 the optic vesicle and those derived directly from the hypodermal cells, 



