No. 420.] REGENERATION IN THE EGG. 953 
earliest observation of this sort was made by Haeckel in 1870. 
Pieces of the embryo of a jellyfish were found to produce each 
a new smaller embryo which gave rise to an entire organism. 
This discovery attracted little or no attention at the time, 
because the important theoretical questions that are involved 
were not appreciated. Without treating the facts in the order 
of their historical sequence I shall describe a few other results 
that have a direct bearing on the questions before us. 
The development of isolated blastomeres is one of the sim- 
plest cases of the kind. If the first two blastomeres or cells of 
the egg of the sea-urchin be separated from each other, each con- 
tinues to segment, in most cases as though it were still a part 
ofa whole. There is produced a half blastula open at the side 
that corresponds to the region at which the other blastomere lay 
at the two-cellstage. The half sphere next closes in to become 
a whole sphere of half size, and from this time onwards the 
embryo continues to develop as a symmetrical whole, passing 
through the later stages as does the normal, whole embryo. 
. These results show that after separation of the blastomeres 
each continues to develop for a time in the same way that it 
would have done had it remained in contact with its fellow; 
but the process of half development is only carried through the 
cleavage stages, and then a new change takes place, initiated 
by the closing of the open side, and a new whole of half size is 
‘established. It is this process of closing of the opening and 
the subsequent whole development of the half piece that resem: 
ble the changes that take place in the regeneration of pieces 
of adult animals. 
In other species of se 
behave in a somewhat different way. 
cleavage is not so obviously a half cleavage, 
the first divisions may be more or less like those of a hilasto- 
mere that remains in contact with its fellow, yet there x not 
formed an open half sphere, but the blastomeres close in as 
they are formed. A whole embryo of half size is also pro- 
duced in this case. 
In another sea-urchin, Toxopneustes, 
after separation of the blastomeres the c 
a-urchins the isolated blastomeres 
In Spheerechinus the 
or at least while 
it has been found that 
leavage may proceed 
