Slonaker, Eye of the Mole. 339 



the anterior end of the neural tube. The optic vesicles con- 

 tinue to increase in a normal way until the embryo has reached 

 a length of about 3 mm. His further description is as fol- 

 lows: "It is interesting to note that for a considerable period 

 after this stage the optic vesicles show but very slight advance- 

 ment to the condition then attained ; their growth appears now 

 to be retarded in as marked a degree as it was advanced in the 

 early stages .... the sudden checking of the development 

 in the mole we may expect is due to the specialization of this 

 species. Any modification of an important sensory organ 

 would doubtless rapidly affect the development of the organ, 

 but such an extended modification as is apparent here says 

 much for the primitive nature of the habits of the animal." 



We thus see that in" the development of the eye of this 

 species there is a very early departure from the normal devel- 

 opment. It is therefore not surprising to find the eye of the 

 young at birth rather poorly developed compared with the 

 advancement found in most mammals at the same period. 



The conditions found in the eye of the adult European 

 mole and our common mole are so similar as to warrant one 

 predicting that the early embryonic stages would be also much 

 alike. 



Tlie Eye at BirtJi. 



The eye is easily seen as a small dark area beneath the 

 skin. The integument separating it from the outside, which 

 conesponds to the eye lids, is 31 mm. thick. The eye lids 

 have fused together in such a manner as to leave only a small 

 cylindrical opening (Figs. 4 and 8, cl). This opening is so 

 small that it is not perceived by the unaided eye. It is there- 

 fore very doubtful if light ever enters it. The cleft also meets 

 the eye at such an angle as to preclude all possibility of light 

 entering along the axis of virion. Exteriorly this cleft appears 

 as a slight funnel-like depression with a small darker center. I 

 have been unable to open it sufficiently to see the eye from the 

 outside. In cross section of the head one is impressed by the 

 small -size of the eye. It has an average equatorial diameter 



