58 'Journal of Comparative Neurology and Psychology. 



gradually decreasing number of buds from the area in front of 

 the first gill back to the fifth. These buds are arranged as 

 follows in N: The area in front of the hyoid contains four buds, 

 in front of the first gill arch 3, in front of the second 2, and in 

 front of the third and fourth one each. The same decrease is 

 present in this area in U. 



The region of the fifth gill arch is confused by the presence 

 of the inferior pharyngeal teeth so that it is difficult to follow 

 the ridge in this region. It is still further complicated by the 

 fact that the basibranchials overlap in the second, third, fourth 

 and fifth arches. Figs. 2 and 3 (Plate I) show these relations 

 schematically. The diagram is drawn to scale and shows not 

 only the segmentation of the median ventral ridge in N and U, 

 but shows the extent also to which the basibranchials overlap. 

 For instance, in the third gill arch, the attachment of the gill 

 extends from section 171 to 205, and the fourth basal-branchial 

 begins at 190 (Plate I, Fig. 3). The portions of the mid-ventral 

 ridge which belong to the third arch are those extending from 

 section 177 to 188 and from 192 to 197. The presence of the 

 portion of the medial ventral ridge belonging to the next gill, 

 namely, from 200 to 206, while apparently on the chart occupy- 

 ing the same area as the third gill arch, really lies parallel with 

 it on the longitudinal axis, so that no difficulty is experienced 

 in locating buds belonging to the third arch. 



It is possible that this arrangement of the buds on the ridge 

 is correlated with the distribution of the twigs of the ninth nerve 

 in front of the attachment of the first gill arch and similarly 

 with the two twigs from the branchiomeric divisions of the tenth 

 in succeeding arches. However, it would be impossible to pre- 

 dict which twigs of these two nerves supply the anterior and 

 posterior divisions, respectively. 



As mentioned in discussing the preceding group, no buds spread 

 from the median ventral ridge to the exterior; in fact, the median 

 ventral ridge is bounded on either side in the later stages by 

 the buds described in the preceding group as lying on the median 

 lateral portion of the copula. 



15. THE OESOPHAGEAL GROUP. 



Buds appear first in the oesophagus in series S, where there 

 are two, and increase to five in series P and to 22 in U. In the 



