1889.] Embryology. 929 
as well as the discovery made by Mr. Eigenmann that the ovum of the 
viviparous surf-perches is almost completely yolkless, resulting in a re- 
duction in volume of the whole ovum to such an extent as to be par- 
alleled in size only by the ova of the more prolific lower forms and 
those of mammals, goes far toward giving us the requisite data for a 
reinvestigation of the causes leading to the diminution and loss of 
the food-yolk, after the latter had once been acquired, as must have 
been tlie case in Pcrlpatus, Micrometrus, and the Mammalia. 
The ova of primitive types were almost wholly without yolk. The 
surplus nutriment of such forms was at once elaborated into a great 
number of small ova, so that the chances of survival were augmented 
by the great fertility thus attained by a species, which was without the 
ance of food, the individual ova of the same species would either tend 
to be multiplied in number in the ovary, or the individual ova would 
tend to increase in size, it might even happen that they would become 
the depositories in which surplus oils or other hydrocarbons would be 
stored up, as actually liappens in the case of fish eggs, thus leading 
through a common histological process, such as that witnessed in the 
formation of ordinary adipose tissue, to the evolution of an egg capa- 
ble of floating at the surface of the water as happens in the case of 
pelagic fish ova. Such a result doubtless would contribute powerfully 
toward favoring the survival of a species provided with such floating 
ova, which we thus perceive may have arisen as a consequence of the 
action of natural causes, not having specifically as their end the salva- 
tion of the species, but rather the disposition of a surplus of material 
elaborated by the female organism and sent to the ovary. 
If, further, the parent female organism became more highly develop- 
ed, more intelligent, circumspect, and alert, the ability to obtain 
nutriment would be increased, but with this increase of powers the 
ovary becomes, as a matter of fact, reduced in dimensions, and there 
is further nothing to prevent one's assuming that the most favorably 
situated ova would receive the most nutriment and reach the largest 
size. Diminution of the ovary would tend to limit the number of 
ova to be nourished, and thus increase their size. If, furthermore, the 
female parent became circumspect, the tendency would be to retain the 
eggs in the oviduct until a favorable opportunity was offered for their 
deposit. If such retention were prolonged, as occurs in reptiles, for a 
considerable time, there would first of all tend to be deposited album- 
inous or plasmic secondary deposits, or secondary membranes, or even 
Am. Nat.-October,-6. 
