648 HAROLD HEATH, 



the median ventral tract is produced by a new process 

 this does not change its j)rimitive character. It re- 

 mains non-segmented while the remainder of the 

 somatic plate possesses an opposite character. So that 

 essentially the same conditions are realized as those which would 

 occur in the larva with the slower more roundabout form of devel- 

 opment. 



From the ectoderm metamerism extended into the middle germ 

 layer and we now have a theoretical type of larva which is realized 

 in the Annelid trochophore. 



According to this idea the ciliated ventral tract in the Annelids, 

 from which the ventral nerve cord arises (Bauchfurche), is the homo- 

 logue of the ventral, ciliated, non-segmented portion of the first somato- 

 blast in Ischnochiton. Consequently it has no more to do with the 

 mouth than any second or third quartette cells of other quadrants. 

 In the earlier stages of the development of Ischnochiton both the 

 second and third quartette cells extend between the prototroch and 

 the blastopore and each contributes equally in all quadrants to the 

 formation of the stomodaeum. And as has been shown in the foregoing 

 pages this is not a phenomenon peculiar to Chiton but belongs also 

 to Annelids and probably to Gastropods, which gives strength to the 

 view that the mouth in Ischnochiton is not a secondarily modified 

 structure. In this form for some time after the mouth commences to 

 migrate to its final position the posterior tertiary stomatoblasts may 

 be seen on its borders, and after these have disappeared the various 

 quartettes retain relatively the same position until it has shifted at 

 least 30". Beyond this point the cells of the first somatoblast become 

 indistinguishable from those of the neighboring third quartette; but 

 it may be safely affirmed that the entire system of stomodœal cells 

 migrates at least one third of the distance to the final position of 

 the mouth. 



In many Annelids the posterior borders of the blastopore con- 

 cresce, as this latter migrates, producing the so called Gastrula-raphe 

 which coincides with the Bauchfurche. In such cases , after the 

 shifting of the embryonic axes is completed, the fused lips are said 

 to extend from the original position of the blastopore along the Bauch- 

 furche terminating in a non-fused portion, the permanent mouth. The 

 Gastrula-raphe, however, occurs entirely within the region of the first 

 somatoblast, while the remaining second and third quartette products 

 in AinpUtrite, as Mead's figures show, behave precisely as in Ischno- 



