500 The American Naturalist. [May, 
X-shaped bands, or in multipolar spindles. In some, the appearances 
point to an active migration of the chromosomes inducing a stretching 
or dragging of the achromatic filaments. 
Many remarkable perturbations appear in the formation of the polar 
bodies. Contrary to the rule, in Ascaris the first polar body may 
divide after its extrusion. The polar bodies may be exceedingly large, 
appearing like blastomeres, and contain more than their share of chro- 
mosomes. In one case the polar body had taken all of the eight chro- 
mosomes, leaving the egg with the sperm only. 
The pronuclei are increased in number when the chromosomes that 
should enter the polar body remain behind in the egg, since they are 
modified into small nuclei. 
In the first cleavage spindle the number of chromatic elements may 
be greater than normally results from the fusion of one male and one 
female pronucleus. 
It is thus evident that very abnormal processes may take place in 
the eggs of Ascaris when exposed to low temperatures. 
In attributing so much to the action of cold it must not be forgotten 
that many such abnormalities have been found in eggs that had never 
been exposed to such temperatures; it is difficult to say just what are 
the limits of the “ normal” processes occurring under the average condi- 
tions. 
Isolated Blastomeres in Ascidians.—Hans Driesch? has ap- 
plied his experimental methods to the eggs of the Ascidian Phallusia 
mammilata and found here, as in the echinoderm, that an isolated 
blastomere may form a complete individual. 
When the eggs are shaken in water for only twenty-five seconds 
some of the blastomeres are so changed that they die and remain as 
inert masses inside the egg membrane, while the other blastomeres 
continue to develop. In this way a complete larva may be formed 
within the egg membrane and adjacent to the dead blastomeres. 
Such larvæ arise from one of the first two cleavage cells and are 
about half the normal size. Otherwise they are like the normal larvæ 
in being perfect and complete individuals, except that the sense or- 
gans and adhesive organs may be in part deficient, as is the case in 
larve reared from whole eggs when exposed to adverse circumstances: 
The larvee are not at all half individuals but whole individuals. 
In the cleavage of these separate blastomeres there is never any ar- 
rangement of cells to represent half the normal state: the cells form a 
"Arehiv. Entwicklungsmechanik. March 8, 1895. 
