150 DEVELOPMENT OF AMPHIOXUS. 



base of the pharynx between the two rows of gilt-slits, 

 but does not reach the posterior end of the pharynx until 

 a much later period. 10 



The features in the development of the endostyle which 

 ought to be especially emphasised are, firstly, its direc- 

 tion of growth from before backwards, and secondly, its 

 primary anterior position in the wall of the pharynx in 

 front of all the gill-slits. 



In connexion with the modification in the shape and 

 position of the mouth, as described above, it is important 

 to insist on the fact that the mouth of the larva is directly 

 converted into the velum of the adult, while the oral hood 

 which grows over the mouth is a new formation. 



During the period of the metamorphosis the larva does 

 not increase in length. It is rather a readjustment of 

 parts which is then taking place than an increase in bulk 

 which is the symbol of active growth. From the time of 

 the first indication of the secondary slits (Fig. 74) till 

 after the completion of the passage of the primary slits 

 from the right to the left side of the body, the average 

 length of the larva may be taken as approximately 

 3.5 mm. 



The adolescent period is essentially the period of active 

 growth in bulk and maturity. The increase in length 

 during this period does not, however, depend on the 

 addition of new myotomes to those already formed, but 

 merely on the progressive growth in size of the latter. 

 The full complement of myotomes was developed during 

 the early larval period, and is present in the larva repre- 

 sented in Fig. 74. 



