DEVELOPMENT. 



419 



The adult position of the anus and of the olfactory pit, both 

 to the left side, and the position of the unpaired liver diver- 

 ticulum, show how partial this process is. 



Experimental Embryology. — As an illustration of what may be called 

 experimental embryology, and of the developmental potentiality of 

 the first few segmentation cells, reference may be made to the recent 

 experiments of Prof. E. B. Wilson. 



By shaking the water in which the the two-celled stages floated, Mr. 

 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 g^strula 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 

 blastula?, gastrula;, and even larvae. Separation into two pairs of cells 



Tf V ge 



^fc***^^ 



Fig. 136. 



-Three larval stages of Amphioxus. (After 

 Ray Lankester and Willey.) 



A, The metapleural folds still separate; />, united posteriorly; 

 C, united altogether ; ap^ atriopore ; gc^ gill slits ; If, left meta- 

 pleural fold ; rf, right metapleural fold ; ?«, mouth ; co, ciliated pit. 



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

 blastulte 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 



