120 
J. T. CUNNINGHAM. 
by the contraction of the protoplasm of the plug of impregna- 
tion, the fibrils of -which run into the interior of the egg. 
III. — Formation of Polar Globules. 
Here again the author gives a critical review of the litera- 
ture, of which the chief points are : that polar globules are 
formed in vegetable ova; that in Toxopneustes lividus 
they are formed before the ovum leaves the ovary ; that, ac- 
cording to Calberla, they are formed in Petromyzon, at the 
metamorphosis of the Ammocoetes; that in Ascidians the 
germinal vesicle disappears in the ovary, and polar globules 
have not been found in their ova ; and lastly, that in Verte- 
brata true polar globules have never been discovered. . 
The view that the polar globules are cells is supported by 
the division of the first globule which was observed by 
O. Hertwig, and still more by the discovery of Trinchese 1 and 
Blochmann 2 that in this division the phases of karyokinesis are 
again exhibited. 
The stage which previous researches left most obscure was 
the transformation of the germinal vesicle into the so-called 
directive spindle. Trinchese and Blochmann state that the 
equatorial plate is derived from the nucleolus of the ovum. 
Trinchese considers the achromatic spindle as the vesicle trans- 
formed, while Blochmann gives no opinion on the point. In 
Vertebrata, except by Hoffmann in Teleosteans, nothing re- 
sembling a directive spindle has ever been found. 
Formation of first Polar Globule. — The structure 
from which the first polar globule is formed and which is itself 
derived from the germinal vesicle, Van Beneden calls the 
Ypsiliform figure, because it resembles a Greek upsilon Y. 
The figure consists of three branches, each composed of 
achromatic fibrils, and of a number of chromatic globules at 
the point of union of the three branches, together with certain 
other elements all achromatic. The figure, as its name implies, 
1 ‘Acad, dei Lincei,’ t. 7. 
5 ‘ Z. f. w. Z.,’ Bd. xxxvi. 
