1893.] Embryology. 159 
blastomere develops as a unit, not asa half-unit ; and the cells to which 
it gives rise cannot be individually identified with those of a normal 
embryo-half. The development is transformed from the beginning.” 
The second paper, by Hans Dreisch, of Zürich,’ contains many 
important facts not presented in the preliminary article noticed in the 
December NATURALIST. 
When the eggs of certain sea-urchins are exposed to high tempera- 
tures an acceleration in the rapidity of cleavage processes is not the 
only result—but at a temperature of 31° C. certain marked changes 
both in the size and position of cells, the entire absence of micromeres 
and other such fundamental effects are produced. Nevertheless eggs 
that cleave in these abnormal ways may develop into perfectly normal 
larval forms, plutei. Long continued exposure to such temperatures 
acts like the mechanical force applied in shaking eggs in a test-tube 
and results in the separation of cleaving eggs into two, rarely more 
than two portions, each of which may then develop by itself. As 
some of these portions form normal larve, high temperature may be 
one of the factors concerned in the formation of multiple embryos. 
Returning to the cleavage of cells separated by shaking echinoderm 
eggs, a series of figures are given to illustrate the fact that the isolated 
cells continue to cleave, for a time, as if still in combination with their 
fellow cells. Thus three cells taken from a four-celled stage do not at 
once fill up the gap left by the removal of the fourth and this fourth 
cell divides by itself just as it would have done if remaining in contact 
with the three others. It is only at a later stage that the products of 
isolated cells arrange themselves so as to form a complete, symmetrical 
individual. 
A portion of the paper is taken up with certain abnormal methods 
of cleavage, beginning with the simultaneous formation of four instead 
of two cells, at the first cleavage. In these cases the cleavage contin- 
ues to be double, a sixteen-cell stage having two sets of each eight cells 
comparable to the entire eight-cell stage of a normal egg. No larvæ 
could be reared from such eggs, yet special interest attaches to them 
upon the assumption that they owe their dual nature to a double fertili- 
zation. The only evidence that these eggs were fertilized by two 
sperms is the assumption by Fol and others that double fertilization 
causes the formation of four in place of the normal two first cleavage 
A most important addition to the methods of experimentation has 
resulted in unexpected results. It consists in subjecting echinoderm 
*Entwicklungsmechanische Studien. Zeit. f. Wiss. Zool., 55, 1892. . 
