924 ' The American Naturalist. [October, 
a very minute portion of which segments and takes part in the forma- 
tion of the embryo. The egg is deposited by the mother in the water, 
where it lies or swims unprotected. As the tail of the young fish is 
formed it begins to move, first feebly and then vigorously, until the 
membrane surrounding the ^^g is burst and the young fish swims out. 
At the time of hatching the intestinal tract is still very rudimentary, 
the food, until the intestines are more fully developed, being supplied 
by the large mass of yolk stored in the egg. The tail is at this time 
quite free from the yolk, and it is usually fringed by a broad, thin 
The development of Micrometrus differs from the usual mode in 
some very essential characters, as this form belongs to a family of 
viviparous fishes almost exclusively confined to the west coast of North 
America. All the members of this family give birth to their young in 
an advanced stage. Micrometrus has gone further in this peculiar line 
of development than the other members of the family with the excep- 
tion of Abeona minima. The ovary of the former consists of a spin- 
dle-shaped tube, from the dorsal wall of which are suspended six broad, 
thin sheets of membrane, in which are scattered comparatively few 
ova. At the time the eggs are ripe the ovary is no thicker than a 
goose quill, and the oviferous tissues are folded upon themselves. 
With the growth of the embryos the walls of the ovary become greatly 
distended, the oviferous sheets unfolding at the same time.^ 
To follow the development of a single tgg : While still in the tis- 
sues in which it was developed it measures .24 mm. in diameter, is 
opaque, and contains a germinal vesicle. At the time of ripening the 
egg contents shrink to less than half their original volume, the ger- 
minal vesicle disappears, and the protoplasm of the ovum is separated 
from the yolk or food material, the whole contents of the egg measur- 
ing at the end of this shrinking but .18 mm. in diameter. By com- 
paring this diameter with that of the average fish egg, it will be 
noticed that the volume of this is more than 100 times less than that 
of the ordinary fish egg. This reduction is not merely mechanical ; 
it is due to the non -formation of yolk, which has been reduced to a 
small particle lying at one side of the comparatively large mass of 
protoplasm, forming the germinal mass, which segments at once after 
impregnation. 
This non-formation of food yolk may be explained in the following 
manner ; taking it for granted that the embryo will be supplied with all 
the necessary food by the mother. The ovary of the typical fish pass- 
es through a state of physiological rest and a state of physiological 
