VIVIPAROUS FISHES OF THE PACIFIC COAST. 
435 
By the equal growth of the blastoderm over the yolk and the formation of the 
embryo along its posterior part (B) the center of gravity of the blastoderm is changed 
from a point in a line joining C and a (fig. A) to a point in the line joining C'b (fig. B). 
This change of the center of gravity rotates the egg till the line cb coincides with 
the vertical. The oil-globule in the meanwhile shifts its position so that its center lies 
in the extended line joining b and C. The globule must move if its position is not abso- 
lutely fixed, and it certainly is movable in some pelagic eggs, and probably is in Serranus 
atrarius=Centropristis striatus. The rotation of the egg is as gradual as the growth 
of the embryo, and the change of position of the oil-sphere can not be observed on that 
account during natural development. The conditions of diagram C are produced by 
the continuation of the same process. Since the center of gravity of the embryo will 
naturally be somewhere near its middle, the head and tail will be kept approximately 
equidistant from the oil-globule. The center of gravity travels with growth from the 
tip of the head in A to the middle of the body in C , and since the embryo encircles 
about half the yolk the change amounts to about 90°. The center of gravity of the 
yolk being fixed, the egg, which is free to respond to the slightest change, rotates an 
amount equal to the change of position of the center of gravity of the germ. This 
does not seem to have occurred to Wilson, else this explanation would not be neces- 
sary, especially since Ryder gave a similar explanation in 1886, p. 10. 
The above supposition, which is entirely warranted by the conditions figured for 
the bass and by the conditions observed in other eggs, explains why the anterior and 
posterior margins of the blastoderm progressing evenly, the blastopore does not close 
over the oil-globule. It does close over the position originally occupied by the globule 
or very near it. On this supposition the embryo is formed by the continuous concres- 
cence of the embryonic rim, the head being the first part formed and remaining fixed. 
It may be worth stating that, at the beginning of development by the more rapid 
ingrowth of cells at the posterior margin of the blastoderm, the embryo, so to speak, 
is pushed towards the anterior rim of the blastoderm, as in A. But this process ceases 
when the more active epibolic growth begins. 
Now let us examine the explanation Wilson gives for this simple matter, using 
the same figures and diagrams, bearing in mind that he considers the oil-globule a 
fixed point : 
On comparing figs. 35 and 36, it is seen that while the posterior pole of the blastoderm remains 
comparatively fixed, the head end («) of the embryo follows, though at a much slower rate, the 
anterior pole of the blastoderm in its growth round the yolk. 
In other words, if we compare the figures the backward growth of the posterior 
margin of the blastoderm plus the anterior growth of the embryo represented by xa' 
equals the advance of the anterior margin of the blastoderm represented by y'y. 
The comparison of the two figures inevitably leads to the conclusion that the increase in length 
which the embryo undergoes in passing from one stage to another is due to intussusception and not 
to concrescence. Extending the comparison to the later stage (fig. 38, just before the blastopore 
closes), it is seen that the increase in length which the embryo undergoes between the stages repre- 
sented by figs. 36 and 38 is brought about in a different way from that between figs. 35 and 36 ( !). 
This is shown by the following examination: At the beginning of the older period (fig. 36) the head 
and tail end of the embryo are approximately equidistant from the oil-globule, and at the end of the 
period (fig. 38) the case is the.same. The head end of the embryo has, therefore, continued to grow 
round the yolk, as in the period figs. 35 to 36, and the body has also lengthened at the opposite end 
in an opposite direction. The increase in length at the tail end of the embryo deserves especial atten- 
tion. The great increase in length which the body undergoes by the growth round the yolk of the 
