ANATOMY AND DEVELOPMENT. 229 



is of a very singular character, and it is difficult to define 

 it in precise terms. 



In this way then the endostyle (and branchial sac 

 generally) comes to be placed at right angles to its primary 

 position. 



Since in Amphioxus the endostyle altered its primary 

 axis by a process of independent growth while the long 

 axis of the pharynx was constant throughout the develop- 

 ment, we find that here again, as in so many previous 

 instances, the details by which similar end-results are 

 arrived at are widely dissimilar. 



This complete change of axis by which the praeoral lobe 

 (organ of fixation) becomes placed at the posterior extrem- 

 ity of the body can only be regarded as a cenogenetic 

 feature.* 



It is therefore chiefly to the primary relations which the 

 various structures bear to one another, before the change 

 of axis, that we must turn for purposes of comparison. If 

 we do this, we find that the following sequence of organs 

 obtains as well in the larva of Amphioxus as in the newly 

 fixed larva of Ciona ; namely: i, praeoral lobe; 2, endo- 

 style ; 3, mouth ; 4, gill-clefts. 



Formation of Additional Branchial Stigmata. 



After the change of axis of the body, the long axes of 

 the stigmata lie transversely. In their further growth 

 they go on elongating in the same (transverse) direction, 

 and after they have attained a certain size their ventral 

 ends that is to say, the ends nearest the endostyle bend 

 round towards each other, and from each of the two first- 



* It goes without saying that the primary long axis of the Ascidian larva is 

 homologous with the long axis of Amphioxus. 



