24 THE AMPHIOXUS. 



unbroken succession. Hero, so far as the quantity of 

 material and the similarity of age in the stages were 

 concerned, the conditions were favourable to an extent 

 that is reached only in few other cases, as, for instance, 

 when artificial fertilization is used. 



In the course of the forenoon, it is possible to follow 

 the closing of the gastrnla, and this should be mostly 

 studied in the living object. 



The formation of the first mesoblastic somites which 

 then succeeds is of such rapidity that the process must 

 often be studied in the living object and in preserved 

 preparations in order to understand it thoroughly. 



The development proceeds with such rapidity, and 

 specially oil the first afternoon, that the examination of 

 the object can scarcely keep pace with it. 



Daring the next hours the development becomes 

 more and more slow. After the lapse of about forty- 

 eight hours, we have the formation of the mouth and 

 first gill-slit, and these mark the conclusion of the 

 embryo development. In order to study all the stages 

 previous to this, there must, I think, be for some 

 time night work. Through alterations in the speed 

 of development, this speed depending on differences 

 of temperature, it is quite true that with repeated 



