DEVELOPMENT OF AMPHIOXUS 165 



way down the body, just ventral to the nerve-tube. As the 

 embryo grows in length, new cells are added on to the noto- 

 chord rudiment from behind by the activity of the rim of the 

 blastopore. 



In each of the bands of cells which will give rise to the 

 mesoderm, a longitudinal groove develops ; the groove 

 opening widely into the cavity of the archenteron. The 

 grooves deepen, and their front portions become separated 

 from the more posterior region by a transverse partition on 

 each side. These front portions become cut off from the 

 archenteron altogether, and so a pair of mesodermal pouches 

 are nipped off, each containing a portion of coelomic cavity 

 which has been in communication with the archenteron and is 

 therefore called an enteroccel. This pair of pouches gives 

 rise to the first pair of somites, and it must not be mistaken for 

 the pair of anterior head-cavities or anterior gut- diverticula, 

 which develops farther forward and at a later stage. 



Behind the first pair of somites, the grooves become nipped 

 off from the cavity of the archenteron anteriorly, while they 

 continue to communicate with it posteriorly. This means 

 that the mesoderm becomes separated from the wall of the 

 archenteron progressively from in front backwards ; and it 

 also becomes divided up by transverse partitions into somites 

 from in front backwards. These posterior somites (from the 

 second inclusive) differ from the first pair only in that the 

 mesoderm from which they are formed becomes separated 

 from the wall of the archenteron before being broken up into 

 somites, whereas the first pair of somites is demarcated before 

 losing connexion with the wall of the archenteron. 



The mesoderm is therefore segmented very early, and each 

 segmental block of mesoderm or somite is separated from the 

 ones in front and behind by a septum. The somites in the 

 anterior region are derived from tissue which was invaginated 

 to form the original archenteron, and consequently they are 

 said to be formed from " gastral " mesoderm. The more 

 posterior somites owe their substance to the production of new 

 cells by the rim of the blastopore as the embryo elongates, and 

 such mesoderm is called peristomial. The difference between 



