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, to a 

 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 larvae. 



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 

 blastulae, gastrulae, and oval larvae. Separation into two pairs of 

 cells resulted in two half-sized embryos. Incomplete separation 

 resulted in one of three types — {a) double embryos, (b) 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. Fiat plates, curved plates, even 

 one-eighth-size blastulae were formed, but none seemed capable of full 

 development. 



This is most interesting. A unit from tlie 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 gastrula2 may be produced from two- or four-celled 



