1892.] Experimental Embryology. 583 
Such is the accuracy and delicacy of this apparatus that the 
sea urchin’s egg, only one tenth of a millimeter in diameter, 
was actually pierced by the finer glass needles. 
To begin with this latter experiment upon the sea urchin, 
the needle is followed by sea water, which remains in part 
within the egg when the needle is withdrawn but yet grad- 
ually disappears as the egg closes in over the wound and does 
not afterwards exercise any evil influence upon the subsequent 
development of the egg. An egg entirely pierced from one 
side to the other subsequently formed a normal pluteus. 
In the ascidian, however, the perforation of the egg or of 
one of its cells gives rise to its death. Within a minute after 
the stab is made in the protoplasm of the cell an appearance 
of opacity or turbidity is seen rapidly extending through the 
entire cell; the cell dies. Later the protoplasm coagulates and 
remains fixed in whatever form it was fashioned by the press- 
ure of adjacent cells when it died. This death of the cell by 
stabbing is a different, much more rapid, process from the 
gradual death observed to occur in many eggs of abnormal 
origin. The part of the egg not injured develops, however, 
just as when the death of its fellow cell was natural or of 
unknown origin. In this development certain definite rear- 
rangements of the cells take place, since they are no longer 
held in normal positions by the attraction of the dead cell ; 
then cleavage continues and finally imperfect larve result. 
Some examples of the results obtained may be given here 
in detail. In normal cleavage the egg divides first into 
a right and a left cell, these divide into anterior and pos- 
terior cells, and the four resulting cells then divide equa- 
torially into oral and aboral cells. If, in the two-celled 
stage the left one be killed, the right divides into an anterior 
and a posterior cell as if nothing had happened to the egg, and 
then these two divide as usual but arrange themselves much 
as if the dead cell were not present, pressing together into a 
spheroidal mass so that the posterior. upper and anterior 
lower cells come into contact diagonally on one side next 
the dead cell or median plane, just as do the other two cells on 
the outside or right of the egg. Thus under the influence of 
