VIVIPAROUS FISHES OF THE PACIFIC COAST. 
431 
The methods of concrescence employed by teleosts in general are obscured in 
Cymatogaster beyond recognition. In fact they are slurred over entirely, for just at 
the time when in other teleosts the infolding process and the formation of the embryo 
begin, the blastopore is in the act of closing in Cymatogaster. Though the gastrulas 
appear to have nothing in common, the conditions in Cymatogaster after the embryo 
has been formed can readily be derived from large-yolked teleost eggs by simply 
imagining the yolk to be reduced. Ryder has shown that the arc formed by the larva 
varies inversely as the yolk. For example, in large-yolked species, as the trout, the 
embryo forms an arc of but 90 degrees. From this, as an extreme, there is a complete 
series of intermediate forms till we reach Clupea , in which the embryo, just before the 
tail buds out, encircles almost the entire yolk. In Cymatogaster the head and tail 
overlap just before the tail begins to grow out, or shortly after the time of hatching. 
Bearing these facts in mind, the incongruous gastrula of Cymatogaster becomes more 
intelligible. 
With the reduction of the yolk, unless this is proportionate to the reduction of 
the germ, which it is not, the embryo must necessarily form a greater and greater arc, 
until it finally forms a complete circle. From this it is evident that, although the 
methods employed by Cymatogaster have become greatly modified, the result is per- 
fectly homologous with the result of gastrulation of other fishes. Ziegler* (1882) 
homologizes the yolk and periblast of teleosts with the yolk cells of the amphibia. 
Wilson (1891) indorses this view and explains the steps by which the amphibian egg 
was changed into the teleostean egg in the following words : 
The alimentary canal is formed, from the roof of the archenteron exclusively. How this was 
effected is easy to see. The increase in the size of the mass of yolk cells (of Amphibia) brought it about 
that the dorsal parts of the embryo were early folded off— sometime before the alimentary canal was 
completed ventrally. The division of labor, already far advanced between the dorsal and ventral 
hypoblast of the gastrula, next took the final step ; the dorsal hypoblast assumed the entire function 
of forming the gut, while the ventral hypoblast became transformed into pure food material. The 
yolk is consequently to be looked on as au organ of the gastrula, which has lost its original function, 
but which, in doing so, became adapted to another function to which it owes its large size. 
The major premise in this argument is that the yolk in teleosts is much larger 
than in Amphibia. But is it larger? It is larger in certain forms — Salmo, for 
instanced This accounts for Ziegler’s view, since he studied a large-yolked form. 
It is just as certainly smaller in certain other eggs, Abeona , Cymatogaster , and Clupea , 
and it seems to me as proper to take the extremes in one direction for comparison as 
those in another. If the egg of Cymatogaster , for instance, is compared with the 
amphibian egg, we can not with Wilson say that the teleostean conditions are due 
to “the increase in the size of the yolk cells of amphibia,” for there is a decrease in 
the bulk of the yolk, and therefore in the yolk cells, if the yolk is formed by the union 
of such cells. Of course it may be argued that Cymatogaster is not a fair representa- 
tive of teleostean ova, because a reduction of the yolk has been brought about by 
viviparity. If, however, the teleostean egg has been immediately derived from the 
amphibian egg, through increase of yolk, we may properly suppose that with the 
reduction of this same yolk the egg will show some atavistic features in the partial or 
occasional segmentation of the reduced yolk. But the tenacity with which the egg 
* For this statement of Ziegler’s views I am indebted to Wilson (1891), pp. 264, 265. 
t It is very large in T achy sums. 
