RELATIONS OF AMPHIOXUS AND TUNICATES, 471 
Experimental embryology.—As an. illustration of experimental em- 
bryology, and of the developmental potentiality of the early segmentation 
cells, reference may be made to the experiments of Professor E. B, 
Wilson. 
By shaking the water in which the two-celled stages floated, Professor 
Wilson separated the two cells, and the result was two quite separate 
and independent twins of half the normal size. Each of the isolated 
cells segments (ke 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 incom- 
pletely, double embryos—like Siamese twins—result, and also form 
short-lived (twenty-four hours) segmented larvee. ; 
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 
blastulee, gastrulze, and even larve. Separation into two pairs of cells 
resulted in two half-sized embryos. Incomplete separation resulted in 
one of three types—(a) double embryos, (4) triple embryos—one twice 
the size of the other two—and (c¢) quadruple embryos, each a quarter 
size. é 
Isolated blastomeres of the eight-celled stage never formed gastrule., 
Flat plates, curved plates, even one-eighth size blastulz were formed, 
but none seemed capable of full development. 
Thus a unit from the four-cell stage may form an embryo, but a unit 
from ‘the eight-cell stage does not. For various reasons it seems likely 
that this is due’ to qualitative limitations, not merely to the fact’ that 
the units of the eight-cell stage are smaller. For although the separated 
cells of the eight-cell stage have considerable vitality, and swim about 
actively, the difference between macromeres and micromeres has by this 
time been established ; in fact, the cells have begun to be specialised, 
and have no longer the primitive completeness, the absence of differentia- 
tion, which explains the developmental potentiality of the separated units 
of the two-celled or four-celled stages. 
Somewhat similar experiments have been made by other investigators 
on the developing ova of Ascidians, sea-urchins, etc. Specialisation of 
segmentation cells appears to occur at different times in different animals, 
but it is illogical to infer the absence of specialisation from the fact that 
any of the first four blastomeres, let us say, can produce an entire embryo. 
For specialised cells may retain a power of regeneration. 
RELATIONS OF AMPHIOXUS AND TUNICATES 
The above account of Amphioxus will in its details 
recall to the student the description of Tunicates. It is 
indeed remarkable that the resemblance should be so much 
stronger in minor anatomical points than in broad outline, 
but this is in part explained by the very marked degenera- 
tion displayed by the adult Ascidians, 
