262 BULLETIN OF THE UNITED STATES FISH COMMISSION. 
concrescence. I fully admit 1 and 2, but tbe Bass development shows that the 
primitive streak is the only jjart of the embryo which it is fair to conclude is formed 
from the nou-embryonic germ ring, and hence Cunningham’s conclusion (3) is entirely 
inadmissible. Cunningham’s whole conception of the non-embryonic germ ring is, as 
I shall try to show, an erroneous one. 
Teleosiean gastrulation and the significance of the germ ring. — The Teleostean gas- 
trula is such a complicated embryonic form that it has given rise to many interpreta- 
tions, and the disagreement as to the proper one still continues. So much is this the 
case that the light which the Teleostean development is capable of throwing on the 
embryology of the Amniotic vertebrates has been greatly obscured. I give a brief 
review of the several theories on this head, ending with Ziegler’s, which, besides 
affording a satisfactory explanation of the Teleost embryo itself, makes practicable so 
many comparisons with both Ichthyopsidan and Amniotic embryos that it leaves little 
to be desired. The only obstacles in the way of the theory have been the lack of exact 
knowledge with regard to Kupffer’s vesicle and the meaning of the extra embryonic 
germ ring. 
Haeckel (19), who witnessed the inflection of the blastoderm edge to form the germ 
ring, regarded the Teleostean gastrnla as a true discogastrnla ; that is, he believed, 
the inflected layer met in the center (as in Fig. 8), forming a complete layer beneath 
the ectoderm. The two-layered 
embryo thus formed he called a 
discogastrula, the yolk lyiag(mor- 
phologically) in the gastrula cav- 
ity. Haeckel’s conception is based 
in the first place, as has long 
been recognized, on wrong obser-, 
vations ; the invaginated tissue 
does not form a complete layer be- 
neath the ectoderm, but remains 
incomplete in the center (Fig. 9). 
In the second place Haeckel’s ex- 
planation, as Balfour pointed out 
(Elasmo. Fishes, p. 277), makes 
it impossible to regard the yolk as a part of the embryo, whereas the comparative 
embryology of vertebrates makes it absolutely certain that the yolk of meroblastic 
eggs is a part of the embryo and has been derived from the yolk cells of some such 
form as the Amphibian blastula. While Haeckel’s theory, as he presented it, no 
longer receives any support, it has obviously influenced the views of more recent 
writers, such as Ryder and Henneguy. According to Ryder the Teleost gastrula has 
been derived in the following manner : 
A gradual loading of the entoblastic pole of the blastula (Amphioxus blastula) -with yolk causes 
tbe latter to be constricted 'around its equator iu tbe course of development, thus leading to tbe forma- 
tion of a blastodisc with an inflected two-layered margin. (35, p. 493.) 
The incomplete center of the inflected under layer, to which Ryder gives the 
convenient name of discopore, is “homologous with a circular opening which might 
be produced by a rupture near the center of the inflected entoblast of the gastrula of 
Branchiostoma.^’ It will be seen that while Ryder agrees with Haeckel in believing 
.r 
Fig. 8, Diagram to illustrate Haeckel’s idea of teleostean gastrulation— 
y., yolk; inv. 1., invaginated layer; ec., ectoderm. 
