78 The American Naturalist. [January 
spiral and the bilateral. In Amphioxus individual eggs are found 
that conform to each of these types; other intermediate methods are 
also observed. All these normal modes of cleevage are figured and 
described with great clearness. 
Any attempt to draw a close comparison between the cleavage of | 
Amphioxus and that of the Annelids must fail. The tempting pole 
cells that the text books have inherited from Hatschek’s account ean- 
not be found: “ the pole cells of Amphioxus are a myth.” ; 
By the method of Driesch the cleavage cells may be isolated and 
the subsequent cleavage of these studied. Many important facts result 
of which but a few may be referred to here. 
One of the first two cells, completely isolated, may cleave like an 
egg, form a blastula, gastrula and even larva of the one gill-slit stage, 
all perfect but of half the normal size. If the cell is not completely — 
isolated from its fellow but merely displaced it will develope more 0 
-less separate from its fellow so that all sorts of twins or more or less 
completely doubled, embryos, blastulas, gastrulas and larve result, ‘ 
Here it is of great import to note that the first cleavage of the dit 
placed cells determines the axial relations of the resulting double mon- 
‘sters. “ Even a slight displacement of blastomers in the two-celled stage 
causes a change in the form of cleavage, such that the blastomers of the 
half embryo cannot be identified individually with those of a nor ) 
embryo half. The normal embryo develops as a unit; if it be disturbed 
z be two celled stage, this unity is destroyed and two new units estab- 
ished.” 7 ; 
Moreover it would seem that “ the unity of the normal embryo is 
not caused by a mere juxtoposition of the cells,” .. . . “this unity p 
not mechanical but physislopieal’:. ; . i-*there must bea structural 
continuity from cell to cell that is the medium of coordination and- 
that is broken by mechanical displacements of blastmeres.” 
Returning again to the actual observations. One of the first four 
cells when isolated may cleave:like a whole egg, may form a perfect — 
blastula, gastrula, larva of one fourth the normal size. If the cells of 
the four-cell stage are not completely isolated various double, triple _ 
and quadruple blastulas may arise. ; 
The isolated cell of the eight-cell stage may cleave in a way much , 
