500 



SCIENCE. 



[N. S. Vol. XI. No. 274. 



Ontogenic degeneration. 



The simplification of the eye in cornutus 

 with age has been mentioned in the foregoing 

 paragraphs. The nuclear layei's are thinner 

 in the old than in the young. There is here 

 not so much an elimination or destruction 

 of elements as a simplification of the ar- 

 rangements of parts, comparatively few 

 being present to start with. 



The steps in ontogenic degenei-atiou can 

 not j'et be given with any degree of finality 

 for Ambhjopsis on account of the great varia- 

 bility of the eye in the adult. While the 

 eyes of the very old have unquestionably 

 degenerated, there is no means of determin- 

 ing what the exact condition of a given eye 

 was at its prime. In the largest individual 

 examined the eye was on one side a mere 

 jumble of scarcely distinguishable cells, the 

 pigment cells and scleral cartilages being 

 the only things that would permit its rec- 

 ognition as an eye. Ou the other side, the 

 degree of development was better. The 

 scleral cartilages are not affected by the 

 degenerative processes and are the only 

 structures that are not. The fact that the 

 eyes are undergoing ontogenic degenera- 

 tion may be taken, as suggested by Kohl, 

 that these eyes have not yet reached a con- 

 dition of equilibrium with their environ- 

 ment or the demands made upon them by 

 use. Furthermore, the result of the onto- 

 genic degeneration is a type of structure 

 below anything found in phylogen3^ It is 

 not so much a reduction of the individual 

 parts as it is a wiping out of all parts. 



PLAN AND PROCESS OF PHYLETIC DEGENERA- 

 TION. 



Does degeneration follow the reverse 

 order of development, or does it follow new 

 lines, and if so, what determines these lines ? 



Before discussing this point I should like 

 to call attention to some of the processes of 

 ontogenic development concerned in the 

 development of the eye. There are three 



processes that are of importance in this 

 connection: 1. The multiplication of cells. 

 2. The arrangement of cells including all 

 the processes leading to morphogenesis. 

 Frequently the first process continues after 

 the second one has been in operation. 3. 

 Lastly we have the growth and modifica- 

 tion of the cells in their respective places 

 to adapt them to the particular function 

 they are to subserve — histogenesis. 



Since the ontogenetic development of the 

 eye is supposed to follow in general lines its 

 phylogenetic development the question re- 

 solves itself into whether or not the eye is ar- 

 rested at a certain stage of its development 

 and whether this causes certain organs to be 

 cut off from development altogether. In 

 this sense the question has been answered 

 in the af&rmative by Kohl. Eitter, while 

 unable to come to a definite conclusion, 

 notes the fact that in one individual of 

 Typlilogohius the lens, which is phylogenet- 

 icall}' a new structure, had disappeared. But 

 this lens had probably been removed as the 

 result of degeneration rather than through 

 the lack of development. 



Kohl supposes that in animals placed in a 

 condition where light was shut off, more or 

 less, some of the developmental processes are 

 retarded. In successive generations earlier 

 and earlier processes in the development of 

 the eye are retarded and finally brought to 

 a standstill ; thus everj^ succeeding genera- 

 tion developed the eye less. Total absence 

 of light must finally prevent the entire an- 

 lage of the eye, but time, he thinks, has 

 not been long enough to accomplish this in 

 any vertebrate. The cessation of develop- 

 ment does not take place at the same time 

 in all parts of the ej^e. The less important, 

 those parts not essential to the perception of 

 light, are disturbed first. The retina and 

 the optic nerve are the least affected, the iris 

 comes next in the series. Because the cor- 

 nea, aqueous and vitreous bodies and the lens 

 are not essential for the performance of the 



