152 Mr Mac Bride, Note on the formation of [May 11, 



same appearance as before posteriorly and also in the most anterior 

 sections: but between there is a place where the lumen has become 

 obliterated, so that the front part of the cavity has now no connec- 

 tion with the hinder part, and thus from a forwardly-directed hollow 

 diverticulum of the gut a mesoblastic somite has been cut off. 

 As the larva grows in length this hollow diverticulum grows also, 

 and from its anterior end a succession of somites are segmented 

 off. In the meantime however, owing to the consumption of the 

 yolk, the larva has diminished very much in diameter, and the 

 cavity of the groove which we may term the ccelomic groove has 

 disappeared owing to the apposition of its opposite walls ; conse- 

 quently it appears as a solid outgrowth of the gut -wall, with 

 which however it does not, so far as I have seen, lose its con- 

 nection. 



The mesoblast of Amphioxus may thus be said to originate as 

 one pair of pouches, opening into the gut behind, which gradually 

 become divided into somites ; and it is of great interest to notice 

 that according to recent investigations this is true also of the 

 lower Reptilia. (Will, " Zusammenfassende Uebersicht," Zool. 

 Centralblatt, April 15, 1894.) The vesicles, however, which later 

 become the first pair of myomeres have a different origin. They 

 arise as an independent pair of gut pouches situated anterior to 

 the pouches we have just been considering, and placed also nearer 

 the middle line ; that on the left side retains for some time its 

 connection with the gut ; and although later the continuity of the 

 two lumina ceases to be observable, owing no doubt to the same 

 cause as that which obliterates the lumen of the ccelomic groove, 

 the continuity of the gut and somite walls is traceable in all the 

 stages I have as yet examined. As Hatschek's " nephridium " is 

 found in precisely the same position in the later larva as this 

 neck of communication between the left anterior somite and the 

 gut, I have no doubt that the two structures are identical. The 

 tube known as Hatschek's nephridium is placed in front of the 

 mouth in a horizontal position, and Van Wijhe (" Ueber Am- 

 phioxus," Anat. Anz., 1893) has found that it communicates with 

 the gut; this observation I can confirm, and can add that at 

 its anterior end it opens into the left anterior somite. Van Wijhe 

 imagined that it was derived from the connection between the 

 gut and the left of those anterior outgrowths from the gut, 

 described by Hatschek, which we may term head-cavities ; but 

 these latter soon lose all trace of their primitive communication 

 with the gut. 



On reviewing the facts now related we see first that the 

 ccelom of Amphioxus is an indubitable enteroccele, and secondly 

 that the formation of the mesoblast in this animal is easily refer- 

 able to the type which Bateson has described in the case of 



