EMBRYOLOGY OF THE SEA BASS. 
263 
that the yolk fills the archenteroii, he on the other hand regards the yolk as represent- 
ing a part of the entoderm of the Amphioxtis gastrula. 
Heuneguy’s theory is practically the same: 
La gastrula des poissons osseux est, comme I’a bien vue Haeckel, une veritable discogastrula qui 
par sou mode de formation et par sa coustitution, se rapiirocbe beaucoup plus de la gastrula type de 
I’Ampliioxus que celle des autres Poissous (18, p. 596). Si I’on suppose, en etfet, la blastula de I’Am- 
pbioxus ouverte a sa partie iufdrieure et s’iiivaginant autourd’uue spbbre (vitellus) on aura une image 
exact de la gastrula des Tdleostdens. L’intestin primordial, le protogaster, est rempli par la masse 
• vitelline (p. 597). 
It will be seen that both of these writers refer the Teleost gastrula directly to that 
of Amphioxtis, and accordingly regard the ingrowth of cells round the entire edge of 
the fish blastoderm as representing the invagination of AmpMoxus, the cavity of the 
fish gastrula being filled with yolk, which has been derived from the bottom cells of 
the Ampliioxus archenteron. It can not be denied that this theory offers an explana- 
tion of the early Teleost gastrula (diagram, Fig. 9), but it becomes utterly unsatis- 
disc. 
Fig. 9. — Diagram of early Teleost gastrula— p. p., posterior pole; a.p., anterior pole; p. h., prim, 
hypoblast; arch., archenteron; p., periblast; s. c., segmentation cavity ; dine., discopore ; ec. 
ectoderm; e. e. g. r. (v. mes,), extra-embryonic germ ring (ventral mosoblast). 
factory as soon as what Balfour has called (7) “ the asymmetry of the vertebrate gas- 
trula ” begins to appear in the fish embryo. For the Teleost gastrula of Eyder and 
Henneguy is a symmetrical gastrula, and they are consequently unable to explain why 
it is that (continued) invagination takes place at one pole of the blastoderm (p.p.), while 
the other pole {s.p.) grows epibolically round the yolk. There are numerous other 
difficulties in the way of the theory, which become apparent as soon as the attempt is 
made to derive in detail the older Teleost embryo (Fig. 10, p. 204, and Fig. 65, PI. xcvi) 
from a gastrula such as the theory assumes. But the greatest objections are, first. 
