1897.] The Advance of Biology in 1895. 787 
point to a combined action of epigenesis and evolution in de- 
velopment. 
The limiting size of the egg consistent with development 
was studied by Morgan, who found that one-fiftieth of an un- 
cleft echinoid egg would develop, and by Loeb, who believed 
one-eighth of the total mass of the egg is necessary to the 
formation of the pluteus, while, in the presence of nucleo- 
plasm, the very smallest quantity of cytoplasm is capable of 
growth and organization. 
The theory that development is controlled by responses to 
stimuli was extended by Herbst and by Davenport to particular 
developmental processes. Roux brought forward his observa- 
tions on the migration of isoiated blastomeres with reference 
ot each other—cytotropism ; these migrations resembling those 
of zoospores towards and from each other (Hartog, Sauvageau). 
Advance was made in interpreting, on the ground of func- 
tional activity, the details of the form of the skeleton (Hirsch) 
and especially of the joints (Tornier). 
Teratogenesis—Double monsters were produced in frogs by 
inversion, which mixes up the contents, and in echinoids, by 
immersing the egg in a salt solution and thereby producing 
an “extraovat.” The effects of low temperature upon develop- 
ment were studied in detail upon frogs and the chick; mag- 
netism was shown again to have little or no effect upon de- 
velopment, while electricity has (Windle); abnormal density 
of solutions caused spina-bifida and other abnormalities in the 
tadpole (Hertwig, Gurwitsch). The capacity for development 
of enucleated egg-fragments into which a spermatozoan has 
penetrated was reasserted, as a result of new studies, by Boveri. 
Regeneration.—Progress was made along three lines: the dis- 
tribution of the capacity for regeneration, the comparison of 
regeneration and ontogeny, and the explanation of regenera- 
tion. As for the distribution of the regeneration capacity, new 
cases were described of the regeneration of internal organs 
(spleen of rabbit, liver of mammals)—not subject to accidental 
amputation. Failure to regenerate was reported of the thyroid 
gland and nerve cells in vertebrates. Experiments revealed a 
capacity for regeneration in the nervous system of earthworms, 
54 
