LATER LARVAL DEVELOPMENT OF AMPHIOXUS. 227 



where the endostyle will eventually come to lie; hence the 

 necessity for their closure.^ 



Morphological Importance of the Development of the 

 Endostyle of Amphioxus. 



1. Its anterior position in the larva in the part of the alimen- 

 tary canal which is equivalent to the median portion of the 

 " prsechordal " vesicle^ shows its entire homology with the endo- 

 style of Ascidians, which is also situated in the median portion 

 of the prsechordal vesicle. 



2. Its anterior position also supports the view that the endo- 

 style was primitively differentiated in and confined to the mid- 

 ventral line of the ancestral prsechordal vesicle. 



3. Its oblique position on the right side is secondary, and is 

 due to the rotatory growth which affected the anterior region 

 of the primitive alimentary tract, in correlation with the 

 adaptive forward extension of the notochord. 



4. It enables the morphological median line of the larva to 

 be fixed with absolute precision, and shows that it, viz. the 

 median line, has come to occupy a secondary and temporary 

 position, high up on the actual right side of the larva. 



5. The important fact can thus be established that the intra- 

 buccai orifice of the club-shaped gland is situated to the right 

 of the morphological middle line. 



6. The endostyle of the larva of Amphioxus possesses two 

 distinct halves, fused together posteriorly, but with free anterior 

 extremities. At first the right or upper half is much smaller 

 than the lower portion, and this is only natural when one re- 

 members that most of the other structures belonging to the 

 primitive right side are wholly retarded in their development. 



7. In the larva it is confined to the region of the first myo- 

 tome which is destitute of gill-slits. 



8. The backward growth of the endostyle of Amphioxus has 

 been acquired concurrently with the multiplication of gill-slits 

 in the segmented region of the trunk. 



^ It must be confessed, however, that the meaning of the first appearance 

 and subsequent atropliy of these slits is not quite clear. 



