43 
equilibrium with their environment or the demands made upon them by 
use. Furthermore, the result of the ontogenic degeneration is a type of 
structure below anything found in phylogeny. It is not so much a reduc- 
tion of the individual parts as it is a wiping out of all parts. . 
PLAN AND PROCESS OF PHYLETIC DEGENERATION. 
Does degeneration follow the reverse order of development, or does it 
follow new lines, and if so, what deteimines 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 connec- 
tion. 1.—The multiplication of cells. 2—The arrangement of cells, in- 
cluding all of 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 modification of the cells in their respec- 
tive places to adapt them to the particular functions they are to subserve— 
histogenesis. Since the ontogenic development of the eye is supposed to 
follow in general lines its phyletic development, the question resolves 
itself into whether or not the eye is arrested at a certain stage of its de- 
velopment and whether this causes certain organs to be cut off from de- 
velopment altogether. In this sense the question has been answered in 
the affirmative by Kohl. Ritter, while unable to come to a definite con- 
clusion, notes the fact that in one individual of Typhlogobius, the lens, 
which is phyletically 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 every suc- 
ceeding generation developed the eye less. Total absence of light must 
finally prevent the entire anlage of the eye, but time, he thinks, has not 
been long enough to accomplish this in any vertebrate. 
The cessation of development does not take place at the same time in 
all parts of the eye. The less important, those not essential to the percep- 
tion of light, are disturbed first. The retina and the optic nerve are the 
last affected; the iris comes next in the series. Because the cornea, 
