DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 



147 



DEVELOPMENT AND LATER HISTORY OF THE EYE OF AMBLYOPSIS. 



The present chapter describes the developmental stages of the eye of the blind 

 fish Amblyopsis spelaus and gives the history of the eye during growth, maturity, 

 and old age. Questions of special interest in the history of this very degenerate 

 organ are : 



i. Do the rudiments of the eye appear as early as usual or later? 



2. How much does the eye grow from the time of its appearance? 



3. When does each part of the eye reach its maximum (a) in size, (b) in mor- 



phogenic development, (c) in histogenic development ? 



4. When does the eye as a whole reach its maximum development? 



5. Are there evidences of a slowing down of the rate of the developmental 



processes : (a) cell division, (b) cell arrangement, (c) cell differentiation ? 



6. Are there evidences of a cutting off of late developmental stages, that is, 



are there any parts of the normal eye that are not developed ? 



7. Does the eye develop directly toward the condition of the adult or does it 



follow palingenetic paths and then retrograde to the condition found 

 in the adult? 



8. What parts of the eye degen- 



erate first ? 

 y. What is the comparative rate 

 of the ontogenetic degenera- 

 tive modifications of the vari- 

 ous parts of the eye, and how 

 does their rate compare with 

 the rate of phylogenetic de- 

 generation implied by the 

 structure of the adult eye 

 of Amblyopsis and the dif- 

 ferent stages of degeneration 

 reached by other members 

 of the family? 

 10. Is there any evidence for or 

 against the dictum of Sedg- 

 wick that structures which 

 have disappeared from the 

 adult organization are re- 

 tained in the embryo only if 

 the organ was of use to the larva after it had ceased to be of use to 

 the adult? 



Fig. 56 



(a)' Outline of Head of Embryo between 1.3 and 1.5 mm. long. 



(b) Outline of Brain and Optic Thickening in Mounted Embryo 1.6 

 mm. long, with 4 Protovertebrae (3.30 p.m.. May 5). 



(c) Outline of Brain and Optic Thickening in Living Embryo 1.92 

 mm. long with 10 Protovertebra? (12 p.m., May 5). 



(d) Outline of Brain and Optic Vesicle of Living Embryo 2.4 mm. 

 long with 10 Protovertebrae (12 p.m., May 5). 



EARLrEST STAGES TO A LENGTH OF THREE MILLIMETERS. 



The development of the eye has been followed in several series of living embryos 

 and in sections of these embryos. The earlier stages of the eye as they were 

 observed in the series obtained on May 4, 1901, will be described. 1 Where advisable 

 other series will be described also. The first indications of the eye are seen in living 

 specimens when the embryo is about 1.5 mm. long, at about the time of the forma- 

 tion of the first protovertebra. This size was reached in the present series in 2.5 to 



1 For an account of the general development of this series see p. 95. 



