VIVIPAROUS FISHES OF THE PACIFIC COAST. 
437 
A, B , and C are precisely the same figures except that the vertical axes of B 
and C correspond to the vertical axis of A. They are placed in this position to make 
clear the necessity of rotation of the egg and the change of position of the oil 
sphere, if, as I hold, the anterior and posterior margins travel equally over the yolk. 
The original position of the blastoderm in B and C is represented by x y. (This does 
in no way agree with x y of “36” and “38”). In the development growth takes place at 
the posterior end of the embryo while the tip of the head remains fixed. Since growth 
takes place in this manner the embryo, if the original axis is maintained, comes to 
have a lateral position, and the blastopore will close at the entodemic pole of the egg. 
This explains why the tip of the head is equidistant from the anterior and from the pos- 
terior margins of the blastoderm. But owing to the fact that the germ is heavier than 
y 
the yolk the egg rotates in the direction of the arrow at x, and the oil-globule at the 
same time travels from its original position, indicated by the dotted line, in the direc- 
tion of the arrow. This explains why the oil-globule remains equidistant between 
the head and tail of the embryo. 
Periblast . — So many accounts have appeared during the past eight years describing 
the origin of the periblast, that it is not necessary to open the main question again, 
especially since the later accounts agree in the chief points. The first clear account 
of the origin of the free nuclei was given by Agassiz & Whitman, 1884. Since then 
several observers haye verified their results. Concerning the function and ultimate 
fate of the periblast nuclei there is still some difference of opinion. Henueguy (1889, 
pp. 46-52) has given an excellent resume of these opinions. 
