320 ENTEROPNEUSTA FROM THE SOUTH PACIFIC. 



VI. Posterior Thematic Complex. 



I have already dealt with what I have called the anterior trematic complex 

 (above, p. 309). 



The posterior trematic complex of the Enteropneusta is situated at the posterior 

 end of the collar in the dorsal middle line, and owes its existence to the close association 

 of the posterior neuropore, the collar-pores, and the first pair of gill-pores 1 . It is un- 

 necessary to repeat what has already been said in the account given of Pt. camosa 

 (see p. 253 and PL XXX. Fig. 21) and Sp. alba (see p. 280). 



It is, however, very important to recognise the existence of the posterior trematic 

 complex, the position of which in the Enteropneusta is due to the fact that only the 

 cerebral portion of the central nervous system has been closed in as a medullary tube. 

 Therefore while the mouth has relations with and forms part of the anterior trematic 

 complex, the anus has nothing to do with the posterior trematic complex of the Entero- 

 pneusta. 



As more of the cerebro-spinal axis becomes closed in by the fusion of the medullary 

 folds, the association of pores which primarily constitutes the posterior trematic complex 

 will be dissolved. When the fusion of the medullary folds reaches the anal region, the 

 posterior trematic complex will comprise the association of posterior neuropore and 

 blastopore (primitive anus). 



We find here therefore a clue to the meaning of the neurenteric canal, which is 

 due to the association of posterior neuropore and blastopore, and their inclusion within 

 the medullary folds. 



If there is any truth in what has been said, it is a matter of such importance 

 that it may be stated categorically that the association of posterior neuropore and 

 blastopore which generally leads to the formation of a neurenteric canal, is the posterior 

 trematic complex of the embryos of Vertebrata. 



VII. Stomochord axd Pygochord. 



The presence of these skeletal products of the gut wall is undoubtedly an expression 

 of the chordate strain in the Enteropneusta, but neither the one nor the other can be 

 homologised with the notochord of the Vertebrata. The pygochord being ventral does 

 not burden the question, but the stomochord is quite another matter. Although there 

 is no question of the pygochord being compared directly with the notochord. yet I 

 think it can be made very suggestive in any attempt to explain the latter. 



The position of the stomochord has been compared with the forward extension of 

 the notochord in Amphioxus. I am convinced that this comparison cannot be wholly 

 sustained because the post-cerebral limitation of the notochord as seen in the Urochorda 

 is undoubtedly more primitive than the cephalochordate condition of Amphioxus. Never- 

 theless, both in Amphioxus and in the embryos of Craniota there is frequently found a 

 disturbance of some kind at the anterior end of the notochord, and this may be due to 

 a local reminiscence of a stomochord. 



1 To these must be added, in Spengelia, the truncal pores. 



