226 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
from a comparison of Teleosts with Amphibia, warrant us in regarding the closure of ' 
the blastopore as a process of concrescence, the result of which is to establish the 
primitive streak. Schwarz has come to the same conclusion (29). 
At the final moment of the blastoi)ore closure there is added to the primitive 
streak the mass of cells, sec. c. m., Fig. 65. This mass is of course the remnant of the ; 
extra-embryonic germ ring. Even after its fusion with the primitive streak, a divid- 
ing furrow makes it for some time recognizable. It may be called, for convenience 
of reference, the secondary caudal mass. 
The entire mass of undifferentiated cells left at the tail of the embryo after the 
blastopore closes, serves as cellular material for the backward growth of the several 
organs. Thus, while the extra-embryonic germ ring, as has been insisted upon by 
Agassiz and Whitman (1. c.) and Cunningham (8), assuredly forms part of the embryo, 
it does not form any special part, but, on the contrary, its cells eventually find their 
way into ectodermic, mesodermic, and notochordal tissues. 
The behavior of the periblast during the final moments of blastopore closure 
may be gathered from Fig. 65, PI. xcvi. As will be seen, the periblast closes over the 
blastopore area before the blastoderm proper. After the closure the periblast plug 
('P- pi-) disappears, the layer forming an even and complete investment of the yolk. i 
Entoderm . — After this digression the early history of the secondary layers and 
the notochord may be resumed. Four hours later than the stage last described (Fig. 36, ji 
PI. xoii, and Fig. 56, PI. xcv) the entoderm is completely established as a connected if 
unicellular layer. In sections through a small embryo like the Bass it is often impos- 
sible to decide whether the entoderm on each side grows under the notochord, as 
shown in Pig. 56, or whether it is split off from the base of the group of chorda cells. 
In order to reach a decision I was forced to cut a great number of sections, for it is 
only now and then that one is obtained in which all the lower cells are clearly defined. 
In the sections through the stage of 25 hours (Fig. 37, PI. xcil, and Figs. 55 and 56, i 
PI. xcv) the extremely intimate connection between the entoderm and chorda was 
uniformly evident, and in some sections, for instance the one given in Fig. 56, it was 
iucontestible that the entoderm was actually growing under the chorda cells. The 
exact state of affairs in the earlier stage (Fig. 53, PI. xciv, and Fig. 54, Plate xcv) was 
likewise often difficult to determine, but the best sections were such as I have drawn. 
After a careful study I feel safe in saying that the lateral sheets of entoderm grow 
under the chorda cells and meet in the middle line, thus completing the layer. Agassiz ' 
and Whitman state the same for Ctenolabnis. In the Trout, according to Henneguy 
(18), this method is not followed. The primitive hypoblast there breaks up into meso- 
blast, notochord, and definitive entoderm, the entoderm being from the first contiuu- , 
ous across the median line. The condition in the Trout is of course a derived one, 
while the development of the Bass in this matter closely follows the ancestral 
lines: the chorda cells at first actually roof in the archenteron along the median dorsal I 
line, as in AmpMoxus and Amphibia. 
The entoderm, after it has grown under the chorda, is shown in the longitudinal || 
section. Fig. 59, PI. xcv, and in the transverse sections. Figs. 60, 61, and 62, PI. xcv, all ^ 
from the same stage (29 hours). The cells of which the layer is composed are flattened 
except at the posterior end of the embryo, in the region of the ueurentric streak (Fig. 
60, PI. xcv, and Fig. 59, PI. xcv, n-m). Since the notochord does not extend into this 
region, it is evident that the entoderm cells here must have had a different mode of 
origin from that employed along the notochordal tract; and in fact they are merely 
