56 W. B. SCOTT AND HENRY F. OSBORN. 



arches, and show narrow cavities lined by columnar epithe- 

 lium (PL XXI, fig. 12, o pp). They present no features of 

 especial importance. We have not followed out the subse- 

 quent development of these segments, but in all probability 

 their cells become transformed into muscle cells. 



In the foregoing description there will be observed a very 

 close similarity to what has been described for the Elasmo- 

 branchs j in fact^ with some minor exceptions, and the one 

 important one of the non-communication of the first pair of 

 segments, Mr. Balfour's descriptions will apply equally well 

 to our specimens. This is of the more interest, for 

 Triton in this respect is very much more like the Elasmo- 

 branchs than it is like the Batrachians ; a fact which is 

 somewhat remarkable. In the Batrachians so carefully in- 

 vestigated by Dr. Gotte,^ there appears to be no head cavity 

 formed at any period. On the other hand, two series of 

 segments, an inner and an outer series, become formed, and 

 are believed by Dr. Goite to correspond respectively to the 

 vertebral and lateral plates of mesoblast which are developed 

 in the trunk. The internal segments resemble the proto- 

 vertebrse in shape, but are smaller; their walls develop into 

 muscular fibres and represent the anterior continuation of 

 the dorsal muscles. The external segments are contained in 

 the visceral arches. In the anterior division of the head 

 (Gotte's Yorderkopf) there is only one pair of segments, as 

 the division of the segment in front of the first visceral cleft 

 does not seem to occur ; the part contained in the mandibular 

 arch is derived from the growth of the postero lateral seg- 

 ments. The most anterior segment of all gives rise, as in 

 the Elasmobranchs, to the muscles of the eye. 



It is remarkable how very different all this is from the 

 process observable in Triton. There are found in the pos- 

 terior part of the head four segments which give rise to 

 muscular fibres, as in Bombinator, and continue the dorsal 

 muscle forwards. These may be equivalent to the four in- 

 ternal segments of the head of Bombinator, but they have no 

 ventral continuations. They are more to be compared with 

 segmentsin the posterior part of the head of the Elasmobranchs, 

 With regard to the latter, Mr. Balfour says (p. 209), ''All 

 my efforts have hitherto failed to demonstrate any segmen- 

 tation in the mesoblast of the head other than that indi- 

 cated by the sections of the body-cavity before mentioned, 

 but since these must be regarded as equivalent to muscle 

 plates any further segmentation of mesoblast could not be 

 anticipated ; to this statement the posterior part of the 

 1 Uiike, pp. 203-208, 21G-229. 



