482 L. S. STONE 



Another individual was operated on at stage 24 and the crest 

 cells along with the upper half of the neural canal were removed 

 from the hyoid and mandibular regions. There was evidently a 

 regeneration of the crest cells, but the specimen showed certain 

 deficiencies in the visceral skeleton. At a level with the optic 

 nerves a section (fig. 81) shows a small quadrate cartilage on the 

 operated side. In levels above and below this section the dif- 

 ference in the size of the two cartilages is even more striking. 

 The anterior portions of the trabeculae show a difference in size. 

 The anterior portion of the trabecula on the normal side extends 

 somewhat beyond the level of the optic nerve, while on the 

 operated side it extends only to the posterior border of the optic 

 nerve. The ceratohyoid cartilage is poorly developed and joins 

 with the first basibranchial posterior to its normal position. 

 The mandibular cartilage in this individual also shows a defi- 

 ciency in size. 



It was found in the large number of operations made in re- 

 moving the neural crest that the most favorable stage was around 

 stage 26. However, this applies only to the branchial and hyoid 

 regions. Here their removal is a very simple matter and no 

 damage to the mesoderm need be expected, for the crest cells lie 

 loosely upon the mesoderm and do not extend very far ventrally. 

 In the mandibular region the eUmination of crest cells is raore 

 difficult, for several factors are involved. From stage 23 to 26, 

 when the regeneration of the neural crest is most persistent, a 

 very rapid ventral growth takes place far in advance of the ven- 

 tral proliferation of crest-cell groups in the hyoid and branchial 

 regions. The crest cells lie tightly against the mesoderm of the 

 mandibular arch, and in order to remove these cells considerable 

 damage is caused to the mesoderm. The wound is very exten- 

 sive ventrally, and the pulling of the cut edge of the ectoderm 

 near the optic vesicle, accompanied by the large cavity made in 

 the anterior portion of the brain, always resulted in the death of 

 such individuals. It is hoped that by doing a large number of 

 operations in this region a few specimens may survive which will 

 show a complete removal of the crest cells over the trigeminal 

 region. 



