1893. EXPERIMENTAL EMBRYOLOGY. 301 
conclusion, into the demonstration of which we shall not enter, that 
the influence on the developing ova was not directly chemical, but 
was due to the altered osmotic pressure of the sea-water. The 
changed chemical composition produces a changed osmotic pressure, 
this acts as a stimulus on the cells of the larva in such a way that 
they are diverted from their ordinary course of development. 
IX. Mr. Edmund B. Wilson, of Columbia College, New York, 
who is well known for some valuable contributions to the embryology 
of Invertebrates, has recently published a preliminary account of 
experiments on the developing eggs of Amphioxus. 
It is well known that the first stages in the development of 
Amphioxus are simple. The fertilised egg divides into two, into 
four, into eight cells, and so on until a blastula is formed whose cells 
are slightly larger in the lower hemisphere. By invagination the 
blastula becomes a gastrula. 
By shaking the water in which the two-celled stages floated, Mr. 
Wilson produced two quite separate and independent twins of half 
the normal size. Each of the isolated cells segments like a normal 
ovum, and gives origin, through blastula and gastrula stages, toa 
half-sized metameric larva. 
If the shaking has separated the two first segmentation-cells 
incompletely, double embryos—like Siamese twins—result, and also 
form short-lived (twenty-four hours) segmented larve. 
Similar experiments with the four-celled stages succeeded, though 
development never continued long after the first appearance of meta- 
merism. Complete isolation of the four cells resulted in four dwarf 
blastule, gastrule, and oval larve. Separation into two pairs of 
cells resulted in two half-sized embryos. Incomplete separation 
resulted in one of three types—(a) double embryos, (0b) triple embryos 
—one twice the size of the other two—and (c) quadruple embryos, 
each a quarter size. 
The eager observer proceeded to shake up the eight-celled stages, 
but in no case did he succeed in rearing a gastrula from an isolated 
unit of the eight-celled stages. Flat plates, curved plates, even 
one-eighth-size blastule were formed, but none seemed capable of full 
development. 
This is most interesting. A unit from the four-cell stage may 
form an embryo, a unit from the eight-cell stage may not. ‘The 
inability to produce a complete embryo may be due either to quanti- 
tative or to qualitative limitations.” Perhaps the one-eighth-cell has 
too little living stuff; perhaps it is already too much differentiated to 
regenerate the whole. We know that there is a size limit to the 
fragments’of Hydra which will regrow an entire organism. 
According to Mr. Wilson, two facts tell against supposing that 
the limit is quantitative. In the first place the one-eighth products 
swim actively, and live as long as the quarter embryos. In the second 
place, minute gastrula may be produced from two- or four-celled 
