RELATIONS OF AMPHIOXUS AND TUNIC ATES. 451 



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 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 incom- 

 pletely, double embryos like Siamese twins result, and also form 

 short-lived (twenty-four hours) segmented larva?. 



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 

 blastulre, gastrulce, and even larvae. Separation into two pairs of cells 

 resulted in two half-sized embryos. Incomplete separation resulted in 

 one of three types (a) double embryos, (6) triple embryos one twice 

 the size of the other two and (c) quadruple embryos, each a quarter 

 size. 



Isolated blastomeres of the eight-celled stage never formed gastrulre. 

 Flat plates, curved plates, even one-eighth size blastula; 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. 



The following important resemblances should be noticed : 

 -In both cases the walls of the pharynx are perforated by 



