302 MEAD. [Vol. XIII. 



taking a small part of the furrow, that between the entomeres, 

 for the whole (cf. Lillie, p. 42). 



Second, during the regional metamorphosis, the "ventral 

 plate " shifts through an angle of about 90°, so that the main 

 portion occupies the ventral region of the trunk, and the telo- 

 blastic area, which lay in the dorsal region near the prototroch, 

 comes to lie at the posterior pole. This leaves the latero-dorsal 

 and mid-dorsal areas to be accounted for in another way. 



From this thesis Wilson deduces his theory of the shifting 

 of the neural axis, and of the homology of the posterior teloblasts 

 with the neuro-nephroblasts of the Oligochaetes and Hirudinea, 

 and elaborates a scheme for harmonizing the apparently irrecon- 

 cilable metamorphosis of various larval forms, — Polygordiiis, 

 Clepsine, Lumbricus, Lopadorhyndms, etc. (Wilson,^ p. 426). 

 The phenomena of axial metamorphosis in Amphitrite are 

 utterly at variance with the terms of this second thesis, and 

 therefore do not substantiate the interpretations which rest 

 upon them (cf. p. 250, text Fig. IV). 



To return to Amphitrite. During the regional changes on 

 the lower hemisphere, the four paratrochal cells, which at first 

 lay in a row at the posterior lip of the blastopore, are brought 

 round to form a circle near the posterior end of the larva, par- 

 allel with the prototroch; and the cells of the somatic plate, 

 concrescing in the mid-ventral line, separate the paratroch from 

 the stomodaeum. From this time the trochophore elongates in 

 the direction of the original egg axis, by the division of the 

 cells of the budding zone, which lie anterior to the prototroch; 

 the latter persists until the trunk develops several metameres, 

 and always belongs to the ultimate segment of the body. 



Few changes take place upon the anterior hemisphere which 

 affect the axial relationships of the larva. The dorsal umbrella 

 mucous glands, whose cytogenetic origin is known, may be relied 

 upon as orienting points until the larva has several setigerous 

 metameres; and the median tentacle, since it bears a constant 

 relation to the mucous glands, orients this region after their 

 disappearance. 



University of Chicago, 

 May, 1895. 



