414 ARTHUR T. MASTRRMAN. 



prevalent in all the Archichorda, a diffuse nervous plexus 

 (stage 1) and sundry definite nerve tracts being character- 

 istic of the group ; whilst in the Euchorda the central and 

 peripheral nervous systems are alike organically separated 

 from the parent ectoderm. 



Applying the same test to the notochord, we find that in 

 the Archichorda a large and more or less indefinite part of 

 the endoderm is metamorphosed into chordoid tissue in 

 accordance with the needs of each particular type. (A cross- 

 section of the pharynx of Balanoglossus shows this 

 clearly.) At the same time there are indications of stage 2, 

 The chordoid tissue becomes definitely located at certain 

 parts, and these become more or less distinct structurally as 

 organs (pleurochords) still in organic continuity with the rest 

 of the endoderm. 



In the Euchoi'da the notochord has completely separated 

 from the endoderm in the adult. 



This method of looking at the matter will show clearly 

 the relationship of the chordoid organs of the Archichorda 

 to those of the Euchorda. It may be possible with further 

 research to indicate an actual organ among the former which 

 may be regarded as the direct homologue of the notochord, 

 but it will probably be in some larval form with an unpaired 

 dorsal rudiment, not in adult Archichorda. 



The same relationships might be pointed out in the meso- 

 derraic organs. The mesoderm in the Archichorda merely 

 shows archimeric segnaentation, and does not show a com- 

 plete separation into various organs. In the Euchorda the 

 mesoderm is not only metamerically segmented, but early 

 divided up into separate oi'gans, such as nephrotome, myo- 

 tome, sclerotome, etc. 



Lastly, in the case of pharyngeal clefts, I have attempted 

 to indicate elsewhere (10b) that the origin of these organs 

 must be sought for in the grooves at the corner of the 

 moutli which, very early even in the history of the Archi- 

 chorda, become segmented backwards from the mouth. In 

 this way I would regard the pharyngeal clefts as originating 



