192 THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 



XV. RETARDATION AND THE CUTTING OFF OF LATE STAGES OF 



DEVELOPMENT. 



In the first paper of the present series (Eigenmann, '99, p. 596) I said: "Cessa- 

 tion of development takes place only in so far as the number of cells are concerned. 

 The number of cell generations produced being continually smaller results in an organ 

 as a consequence, also smaller. In this sense we have a cessation of development 

 (cell division, not morphogenic development) in ever earlier stages. That there is 

 an actual retardation of development is evident from Amblyopsis and Typhlichthys 

 in which the eye has not reached its final form when the fish are 35 millimetres long." 



I am convinced now that this statement does not go far enough. There is, indeed, 

 a gradual retardation in all processes of development which frequently terminates in 

 a complete arrest of development before the final stages of normal eyes are reached. 

 In discussing the changes it will be best to keep the three groups of processes con- 

 cerned in development separate. 



1. Cell Division. The proof of the limiting of the number of cell divisions men- 

 tioned has been brought in the present paper. It has also been seen that the rate of 

 division is very much retarded. In the retina it stops altogether at the time the fish 

 has reached a length of 5 to 7 millimetres, and very rarely more than two dividing 

 cells are found in any eye. In its first stages the eye is thus about equal in size to 

 the adult eye. Cell division stops earlier in the lens where no new cells are formed 

 after it is cut off from the skin. The lens is at this time relatively as well developed 

 as the retina. In both the retina and the lens cell division ceases in late stages and 

 the total number of cell generations is very much limited. The lens is looked upon 

 as phylogenically a new structure, and we have, by the stopping of its later stages of 

 cell division, a step in the elimination of a phylogenically new structure. This is, 

 however, of no consequence because it is not differential, for the retina, a phylogenic- 

 ally older structure, suffers a similar stoppage. There is no evidence, then, that 

 phylogenically younger structures lose their power of cell division earlier than 

 phylogenically older ones. 



2. Morphogenic Processes. The retardation of the morphogenic processes, cell 

 arrangement, movement, union and separation, etc., is conspicuous in the delay of 

 the closing of the choroid fissure and all that this implies. There is no conspicuous 

 stopping of this process except in the occasional failure of the choroid fissure to close 

 at all. 



3. Histogenesis. Histogenic processes are also distinctly retarded, and in con- 

 spicuous instances suffer an entire stoppage. While the eyes of 3-millimetre speci- 



