DURATION OF EMBRYO DEVELOPMENT. 29 



"We have seen that the spawning depends upon the 

 conditions of the weather, and on the temperature. 

 In the same way other influences appear to have 

 an adverse effect. Thus for weeks together I was 

 unable to obtain any deposit of the sexual products 

 from specimens kept in glasses, since, though contain- 

 ing them in abundance, they were plainly under un- 

 favourable conditions. 



B. DURATION OF EMBRYO DEVELOPMENT. 



The development of the Amphioxus may, as I have 

 already said, be divided into two chief parts. The 

 first part may be suitably called development of the 

 embryo, for in this part the development proceeds at 

 the expense of nutriment stored in the egg. 



In this first period the development advances very 

 rapidly. Its duration occupies a period of, on an 

 average, something less than forty-eight hours. That 

 is to say, it continues from the liberation of the eggs, 

 immediately followed by the fertilization, up to the 

 perforation of the mouth and first gill-slit, this in- 

 volving the conclusion of the embryo development. 



The speed with which the processes of development 

 advance in this part depends, for the rest, chiefly 

 upon the conditions of the temperature. I will give a 



