284 Jounial of Comparative Neurology a)ui PsycJwlogy. 



to the adult. The stages of development of the eye are divide;! into 

 four })eriods. 



During the tir.<t period (from appearance of first protovertebrae to 

 embryos 4.5 mm. long) the optic ve.-^icle and lens are formed as in 

 normal embryos, but there is retardation in cell-division and growth. 



In the second period (embryos 5 to 10 mm. long) the optic nerve 

 forms; its diameter is only 12 micra and it does not increase in size. 

 The lens separates from the ectoderm but its cells do not differentiate 

 into lens fibers and degenerate before the end of the period. A rudi- 

 mentary iris forms from the margins of the optic vesicle ; the cavity of 

 the vesicle is practically obliterated, and the choroid fissure becomes a 

 groove which mav remain open. In the retina the pigment layers and 

 inner reticular layer are developed ; outer and inner nuclear layers 

 are not differentiated, nor are the cones or dividing cells present as 

 would be the case in the normal eye. 



The third period (length from 10 to 100 mm.) is characterized by 

 the degeneration of the nerve cells of the retina, the sinking of the 

 eye to a position 5 mm. beneath the surface of the skin, the closure of 

 the pupil and the complete disappearance of the vitreous body. Scle- 

 ral cartilages show progressive development. 



During the fourth period (fish more than 100 mm. long) the 

 scleral cartilages become well developed and the eye muscles show no 

 signs of degeneration. The pigment layer of the retina forms a thin- 

 walled vesicle of considerable size while the nervous layer is less than 

 0.2 mm. in diameter and is markedly degenerate. In one individual 

 observed the eye was completely disintegrated. 



The author concludes "that there is no constant ratio between the 

 extent and degree of ontogenic and phylogenic degeneration." From 

 the rapid degenerative changes observed in ontogeny it is evident that 

 the ultimate fate of the eye of Amhlyopsis is total distinction. 



The incomplete development of the eye is due (i) to retardation 

 and final cessation (jf cell division ; (2) to retardation of morphogenic 

 processes; (3) to the extinction of histogenic activity. All three 

 phenomena weaken as development proceeds. This may be caused 

 by external or internal influences. As, however, the eye remains de- 

 generate in individuals reared in the light, and is well developed in 

 other cave-inhabiting .species, the factor of light may be eliminated. 

 There is moreover, no evidence to show that atrophy is due to pressure 

 from other organs or to lack of nutrition. It only remains to conclude 

 that the causes of the degeneration are inherent in the ovum and are 

 inherited bv the embrvo. 



