584 The American Naturalist. [July, 
mutual attraction the four cells move so that they are arranged 
in a tetrahedron rather than in a square. The next changeis 
a division of each by a plane parallel to the median plane or 
to the face of the dead cell. 
This illustrates a marked tendency in all natural cases of 
death; that is, the planes that normally would be nearly 
meridional, turn so as to become parallel to the dead cell, par- 
allel to a plane passing through the center of the egg. 
From this eight-celled stage the development proceeds till 
a larva is formed, having a normal tail, three distinct germ | 
layers, a pigmental area representing the nervous system and 
one papilla for attachment. Having begun to secrete its cel- 
lulose mantle it died. 
Other cases were obtained showing the same results. Figure 
2, R, R’, R” shows three successive stages in the develop- 
ment of one of these artifi- 
cial right-half embryos com- 
pared with successive stages, 
L, L’, L’’, in the develop- 
ment of a left-half embryo 
of natural or unknown ori- 
gin. 
Again, when the posterior 
left of the normal four cells, 
after two planes have oc- 
curred in diaa iskilled by the needle, an ovoid larva is 
obtained. In this the tail adheres along the trunk and there 
are three papillæ of attachment and two pigment spots. Sim- 
ilar monstrous forms are found when the right posterior cell is 
killed. When the right anterior cell is killed the tail is well 
formed and free from fusion with the trunk, and there are no 
pigment spots or imperfect ones. When the left anterior cell 
is killed the larva has a perfect tail, a papilla for fixation, a 
pigment spot and active movement. It escaped from most of 
the egg membranes and secreted a tunic, into which migra- : 
tory cells were passing when it died. 
Killing two diagonal cells of four is followed by a normal 
cleavage of the two remaining cells, but the experiment was 
FIGURE 2. 
