48 AMPHIOXUS. 



gradually, by increase in the number of the gill-slits and in other 

 ways, acquires the structure and size of the fully formed animal. 



The whole developmental history of Amphioxus may, in 

 accordance with the above account, be conveniently divided into 

 periods, which will be dealt with in succession in the remainder 

 of this chapter. 



I. The Embryonic Period : including the stages from the com- 

 mencement of development to the formation of the mouth. This 

 lasts about thirty-six hours, and is characterised by the extreme 

 rapidity with which the stages, especially the earlier ones, are 

 passed through ; and by the fact that throughout the period the 

 embryo is dependent for nutrition on the yolk granules contained 

 within the egg. The actual rate of development varies to a 

 certain extent with the temperature. The times here given are 

 those recorded by Willey during the summer months ; in spring, 

 Hatschek found the rate of development to be slower. The 

 period may be subdivided into two parts. 



1. Before hatching : from the commencement of develop- 

 ment up to the hatching of the embryo ; a period of about eight 

 hours. 



2. After hatching : from the hatching of the embryo to the 

 formation of the mouth ; a period of about twenty-eight hours, 

 during which the embryo leads a pelagic life. 



II. The Larval Period : from the formation of the mouth to 

 the critical stage. This lasts about three months, and during it 

 the larva is pelagic. Development takes place slowly ; and the 

 most notable events are the formation of the gill-slits and the 

 atrial cavity ; and the curious series of changes by which the 

 symmetrical condition of the larva is re-established. 



III. The Adolescent Period : during which the young Am- 

 phioxus, having adopted the mode of life of the adult, gradually 

 acquires its full structure, by increase in the number of the gill- 

 slits, ripening of the reproductive organs, &c. 



I. THE EMBRYONIC PERIOD. 



From the laying of the eggs to the formation of the mouth. 

 Duration of the period, from thirty-two to thirty-six hours. 



Part I. From the laying of the eggs to the hatching of the 

 embryo : a period of about eight hours. 



