606 E. W. MAOBRIDB. 



appearing behind, and then growing forward ; but the first 

 recognisable trace of the future fold on the right side is an 

 epithelial thickening (fig. 22, a) in the anterior region of the 

 pharynx. This thickening, which later lines the outside of the 

 fold, is recognisable even at the end of the embrj'onic period. 

 It will be remembered that the name " collar cavity " was 

 given to the coelomic pouches so denominated on account of 

 their general resemblance in mode of formation to the collar 

 cavities of Balanoglossus, The homology implied in this 

 name is borne out by the subsequent history of the sacs in 

 question ; for (1) they remain distinct from the cavities 

 derived from the coelomic grooves or trunk coelom, and (2) 

 they swell out into ridges overhanging and protecting the 

 gill-slits, just as the hinder edge of the collar region does in 

 Balanoglossus, as Batesou (2) has pointed out, — only in that 

 animal, of course, at most two gill-slits are protected. 



Resume and Conclusions. 



The most important points established in this paper are as 

 follow. 



(1) The primitive gut or archenteron is formed in Araphi- 

 oxus by a typical process of embolic invagination, the endo- 

 derm being at first not sharply marked ofi* from the ecto- 

 derm. The blastopore is at first posterior, but subsequently 

 becomes dorsal by the preponderant growth of the ventral lip 

 of the blastopore. 



(2) The mesoderm originates in Amphioxus as a series of 

 true gut pouches, viz. one anterior unpaired pouch and two 

 pairs of lateral pouches. Of these the first divides to form 

 the two head cavities : the anterior pair give rise to the first 

 pair of myotomes, and in addition to two long canals extend- 

 ing back ventrally : the posterior pair are gradually separated 

 from the gut, and pari passu divided into a series of myo- 

 tomes. The whole process of mesoderm formation is therefore 

 referable to the type found in Balanoglossus, the main difi'er- 



