REMARKS ON THE DEVELOPMENT OF AMPHIOXUS. 851 



Further Remarks on the Development of 

 Amphioxus. 



Bj 



E. \¥. MacBiide, Itl.A., D.Sc.(I.oii€l.), 



Professor of Zoology in McGill University, Montreal. 



Witli Plate 17. 



In 1897 I published in this Journal (6) a paper entitled 

 " The Early Development of Amphioxus." In this work it was 

 shown — (1) that the gut wall is all formed by the endoderm, 

 and that the blastopore is at first posterior and subsequently 

 displaced on to the dorsal surface ; (2) that the mesoderm 

 originated from five distinct rudiments, viz. (a) an anterior 

 pouch of the gut, from which the head cavities are formed ; 

 (b) a pair of dorso-lateral pouches which give rise to the first 

 pair of myotomes, and to long ventral sac-like outgrowths of 

 the same which extend back into the atrial ridges; and (c) a 

 pair of dorso-lateral grooves in the gut wall extending back 

 to the blastopore, which gradually become separated off in 

 front, and at the same time divided up to form all the myo- 

 tomes which succeed the first. 



It was further shown, in the paper referred to, that the tube 

 found in the larva on the left side above the mouth, and 

 called by Hatschek a nephridium, was the persistent con- 

 nection existing between the first myotome on the left side 

 and the gut, from which in the younger larva it had origi- 

 nated as an outgrowth. Lastly, I asserted that the lymph 

 canals found in the metapleural folds were the remains of the 

 ventral extension of the first myotomes. 



