1893.] Embryology. —— 743 
EMBRYOLOGY.’ 
The Sea-urchin egg.—Hans Driesch’ has recently made an 
interesting addition to his previous work upon the eggs of Echinus 
mricrotuberculatus and Spherechinus granularis by the aid of an 
improvement in his methods. ; 
He reaches the conclusion that by operative interference cells of the 
cleaving egg are made to form entodermal structures though they 
would normally have formed ectodermal structures. 
In his larger paper? the author described the peculiar results obtained 
when eggs are subjected to pressure under a cover glass. 
In such eggs the elastic membrane must be ruptured by pressure in 
order that it may not tend to restore the original spherical shape when 
the pressure is removed, but when the eggs are compressed sufficiently 
to break the membrane many of them are irreparably injured. Sub- 
sequently he found that if the eggs are shaken for 4 or 5 seconds in a 
glass the membrane may be removed from all of them without any 
injury to the eggs. This, however, must be done at the right time, 
about three minutes after the sperm has been added to the eggs and the 
membrane is just plainly separating itself from the egg. 
These membraneless eggs are subjected to the pressure of the cover 
glass when they have divided into 4 cells. Under such circumstances 
they form a disk of 8 cells, since the cleavage spindles now lie horizon- 
tally or at right angles to their normal position. 
When the pressure is removed the 8 cells divide at right angles to 
the plate, forming 16 cells in two layers of 8 each; however there are 
rarely 4 but usually only 2 micromeres formed and these 2 frequently 
lie out at one side of the double plate, not beneath the cells that formed 
tbem. From this double plate of 16 cells there is next formed a 
double plate of 32 cells. The important point here is that, as the author 
maintains, no rearrangement of cells takes place but merely a horizon- 
tal division of each of the 16 to increase the whole to a double plate of 
32, 16 in each layer; yet the cells that gave off the peripherally placed 
micromeres do divide vertically and thus fill the gap in the lower plate, 
which would otherwise remain with only 12 cells. These 12 with, the 
2 vertically formed ] the 2 products of the 2 mi | up 
the 16 of the lower plate. 
1 Edited by Dr. E. A. Andrews, Baltimore, Md. 
2 Anatom. Anz., April 8th, 1893. 
3 See THE AMERICAN NATURALIST for February, 1893. 
