1 8 THE AMPHIOXUS. 



acid to all the embryos, and afterwards carmiii 

 (either Beal's carmin or picrocarmin), then I hardened 

 them in successive strengths of alcohol. Special care 

 should be taken that the application of perosmic acid 

 be not too weak, as otherwise there is an alteration of 

 the forms of the embryo, the epiblast being raised like 

 a bladder from the inner cells. Neither should it be 

 too strong, since in this case the embryos become 

 Liter on so dark in the alcohol that they are practically 

 useless for sectional methods. It is always as well, 

 when dealing with a considerable number of embryos, 

 to test under the microscope some individual speci- 

 mens after the osmic acid and after the carmiii 

 staining. 



Bearing in mind the great speed of development, it 

 is as well to preserve a very considerable number of 

 stages. In the early stages up to that of the ninth 

 mesoblastic somite, I preserved specimens at very short 

 intervals, every half-hour in fact. Later on the inter- 

 vals may be greater, as the embryo itself, in a space 

 of several hours, undergoes only trifling alterations. 



From the stage with mouth and first gill-slit, the 

 embryos were no longer reared in glasses, but collected 

 in the sea. 



