v 



442 L. S. STONE 



cells which appeared in stage 30 over the posterior two-thirds of 

 the branchial region has now split into two bands, the anterior 

 of which is covering the second branchial arch, while the posterior 

 shorter band lies on the mesoderm of the third and fourth bran- 

 chial arches. 



In the lower portion of the visceral arches (fig. 24) the meso- 

 derm of the mandibular, the hyoid, and the first and second 

 branchial arches can be seen on either side entirely surrounded 

 by neural crest. The third branchial arch which appears in 

 more dorsal sections is likewise entirely surrounded by neural 

 crest. As the sections are followed ventrally, many mitotic 

 figures can be seen in these rings of neural-crest cells, indicating 

 a rapid increase in the number of cells as well as rapid migration. 

 They have grown very abundant in their position around the 

 visceral arches, especially so around the anterior portion of the 

 mandibular arch where they are continuous ventrally in front 

 of and behind the stomadaeum upon the maxillary and man- 

 dibvdar processes. 



Placodes. The ophthalmic ganglion, at its anterior extremity, 

 has a wide contact with the ectoderm (fig. 25). Mitotic figures 

 appear on the ectodermal side of the contact throughout this and 

 other sections. The extent of the contact is indicated in figure 6. 

 The gasserian ganglion near its distal end has a definite contact, 

 for about four sections, with the ectoderm at the anteroventral 

 border of the VII lateral-line placode (fig. 26). This is possibly 

 a placode for the gasserian ganglion. The actual contact is very 

 brief and its history is difficult to follow. In later stages it is 

 not possible to detect the placode. Many placodal cells forming 

 the lateral-line ganglion of V^II are splitting off from the posterior 

 portion of the elongated placode which lies above the hyoid arch 

 and hyomandibular cleft (fig. 6). The lower portion of the 

 lateral-line ganglion, which is thus forming, consists of cells less 

 compact at this stage. These later supplj'' the hyomandibular 

 division of the lateral-line component of VII. Where the VII 

 lateral-line placode fuses with the placode in the dorsal portion 

 of the hyomandibular cleft (fig. 6) the anterodorsal extension is 

 larger than in stage 30. In the epibranchial placode of VII are 



