EMBRYOLOGY OF THE SEA BASS. 
267 
our observations are correct” {p. 79). With respect to concrescence this statement is 
made: 
It appears quite certain to ns that the principle of concrescence underlies the formation of the 
embryo. The concrescence appears under the disguised form of a migratory movement of the cells, 
which accompanies the epibolic growth of the blastoderm. (P. 74.) 
Now, before accepting the hypothesis that the extra-embryonic ring represents 
chorda-entoblast, we must be satisfied that the two halves of the ring meet along the 
chorda line. And I have tried to show that in the Bass there is no reason for believ- 
ing this. Until the concrescence is actually proven, I do not think this manner of 
looking at the germ ring is admissible. 
But, if the concrescence theory were established, even then it would scarcely be an 
explanation of the germ ring to call it chorda-entoblast. For at all events a part of 
the ring occupies a ventral position with respect to the blastopore (e. e. g. r., Fig. 10, 
sec.G.m., Fig. 65, PI. xcvi), and it is obviously impossible to regard this part as chorda- 
entoblast. Evident as this would seem to be, Cunningham has been curiously misled 
into regarding the whole ring as equivalent to dorsal hypoblast, because it eventually 
comes to occupy a dorsal position with respect to the yolk. Cunningham’s position is 
fairly given in the following quotations (8) : 
The whole of the embryonic ring thus belongs to, and is formed into, the dorsal region of the 
embryo. (P. 17.) 
It is probable that the inflected ring in the Teleost is the dorsal hypoblast. It has already been 
pointed out that the whole of the inflected ring comes to lie beneath the axis of the embryonic rudi- 
ment, between that axis and the yolk. The invaginated layer thus ultimately occupies the same posi- 
tion as the layer in the blastoderm of the bird, to which the name hypoblast was first applied. (P.20.) 
With respect to the occurrence of concrescence, Cunningham gives no actual evi- 
dence, and there is nothing in his account which would cause me to believe that the 
growth of the embryo, in the fishes he studied, takes place in a different way from that 
of the Bass. The fact that the whole of the germ ring is absorbed into what Cunning- 
ham calls the dorsal region of the embryo (more properly terminal portion) is surely 
no argument that the germ ring is equivalent to dorsal hypoblast. The mor- 
phology of the ring must be determined before the blastopore closes, and an ingrowth 
from the ventral liii of the blastopore can scarcely be called dorsal hypoblast. 
A satisfactory explanation of the extra-embryonic germ ring can only be obtained 
by regarding it as mesoblast. The ingrowth from the dorsal lip of the blastopore 
in the Teleost consists of primitive hypoblast (mesoblast plates, chorda, roof of archen- 
teron). This ingrowth is continuous with that which grows in from the ventral lip, 
and which consists of mesoblast. Precisely the same state of affairs is found in the 
frog (Fig. 11, p. 265), as may be gathered from Oscar Hertwig’s account (20, p. 273) of 
its development : 
Miissen wir sebliessen, dass am Urraundrand der Ektoblast in das lunere der Embryonal form binein 
wuebert, und bier einerseits in einen Streif ibrer dorsaleu Wand ubergebt, der den Harm nacli oben als 
Cborda-Entoblast begrenzt, andererseits sich in den Mesoblast contiimirlich verfolgen Idsst. 
In the phylogeuy of the fish gastrula the entoblast and mesoblast have suffered a 
very similar fate. In the case of the former the ventral entoblast has lost its function, 
the dorsal entoblast assuming the entire duty of forming the alimentary canal. Like- 
wise the ventral mesoblast, which in the Amphibian grows forwards underneath the 
yolk cells and forms the ventral mesodermic tissues of the adult, has in the fish lost 
