428 L. S. STONE 



The dissected embryos were in some cases preserved in glycerin 

 after they had been stained with haematoxylin. However, the 

 neural crest may be more satisfactorily observed unstained in 

 water or alcohol, because its grayish-brown color sufficiently 

 differentiates it from the surrounding tissue up to a certain stage 

 as will be described later. 



Although the picture of the crest cells and placodes is approxi- 

 mately constant for each stage, occasionally normal embryos 

 whose external appearances indicate similar age disclose upon dis- 

 section noticeable differences in the early rate of growth of the 

 crest cells or placodes. This difference may even be confined to 

 the same individual where, for example, the advance of the trunk 

 lateral-line primordia of the one side lags conspicuously behind 

 the primordia of the other side for a distance of one or two 

 somites. 



Stage 21 



Nevral crest. A layer of neural crest, light brown in color, over 

 the dorsal and dorsolateral portions of the medullary tube extends 

 from a point above the eye to a point above the posterior border 

 of the second somite (fig. 1) where it is continuous with the crest 

 cells in the spinal region. Ventrally there are two distinct 

 proliferations separated by a constriction lying approximately 

 midway between the two ends. The more anterior proliferation 

 becomes the band which later wraps around the mesoderm of 

 the hyoid arch. 



Where the dorsal margin of the mesoderm of the branchial 

 region meets the lateral surface of the medullary tube there is 

 a trough-like depression, filled in by a deep longitudinal ridge of 

 ectoderm extending from above the primitive hyomandibular 

 cleft to near the first somite, where it becomes continuous with 

 less marked ridges over the somites. This is represented by 

 dotted lines in the figure. 



Figure 13 is a frontal section passing through the longitudinal 

 ectodermal ridge and the anterior and posterior extremities of 

 the layer of neural crest. The considerable thickness of the 

 ectodermal ridge can be seen on either side in the region of the 



