VIVIPAROUS FISHES OF THE PACIFIC COAST. 
423 
supplied by it with food during all that period. The period of gestation is the period 
of greatest activity — corresponding to the period of greatest growth of ova in oviparous 
species. The reduction of the yolk has taken place after the reduction of the number 
of eggs, as may be seen by comparing the number and sizes of eggs of viviparous 
forms. The reduction of the yolk and of the number of young has evidently gone 
hand in hand with the lengthening of the time the young remain in the ovary. In 
those viviparous species in which the yolk and the number of eggs have not at all 
been reduced ( Sebastes , etc.) the young are liberated as larvae as soon as hatched. 
In those viviparous species in which the young remain for a long period in the ovary, 
from which they derive their food ( JCmbiotocidae , Anableps , Zoarces) the number of 
young has been reduced. In Cymatogaster (and Abeona ), where the reduction of the 
number of young is very great, the average number being but 12, the reduction has 
also been carried to the yolk. 
Though the size of the yolk can thus be satisfactorily explained and charged to the 
account of viviparity, greater difficulty is met when we attempt to determine whether 
other peculiar conditions observed during development, which are due to the small 
size of the yolk, are atavistic, or whether they are further modifications and should 
also be charged to viviparity. To this, however, we shall return later. 
Of especial interest is the yolk nucleus,* since it is also found in Abeona , in 
which the proportions of the egg are similar to those of Cymatogaster. 
Entodermic aggregations of protoplasm have been described by List (Z. W. Z., 
1887, 595) for Crenilabrus tinea , which u zeigt also was die Anordnung der Keim- 
substanz auf dem Dotter betrift grosse Uebereinstimmung mit dem durch Kupifer’s 
Untersuchungen bekannt gewordenen Ei des Herings ” (p. 598). I have not seen 
Kupifer’s work, but have examined the eggs of another species ( Clupea mirabilis), 
which, since the germinal matter is yellow, show the entodermic mass of protoplasm 
to good advantage. In the case of this species, as well as in the case of C. tinea , 
the entodermic mass is not a u selbstandige Masse,” but is merely a thickening of the 
periblast , which extends over the whole yolk and which disappears at the expense of 
the growing germ. But even in these cases this thickening is of doubtful significance, 
and it is still more so in Cymatogaster and Abeona , in which it forms an isolated mass 
sunk deep into the yolk without any apparent connection with the germ, and 
where it remains intact until the closing of the blastopore. It might be possible 
that the entodermic mass has here collected in a space formed in the yolk by the 
phylogenetic degeneration of an oil-globule, if the oil-spheres in teleosts occupied a 
fixed position. But all those that I have been able to examine can change their posi- 
tions without a greater disturbance than would be created by changing a floating- 
ball from one part of a vessel of water to another. The fact also that angles from 
this mass sometimes project in between the surrounding yolk-spheres argues against 
the supposition that it represents a degenerate oil-sphere. On the other hand, Wilson 
has lately shown that there is a cap of protoplasm covering the oil-globule in some 
* This has been identified with the yolk nucleus of ovarian eggs. It is not homologous with the 
entodermic aggregations of protoplasm mentioned in the succeeding paragraph, with which it was at 
first identified. For a history of the early stages of this, see Hubbard, 1894 ; for the later stages see 
paragraphs headed yolk nucleus, p. 440. 
