346 HORACE W. STUNKAED 



bottom of the watch-glass. The neural tube was swept clean 

 of all surrounding tissues by the use of dissecting needles and 

 fine brushes, the sides of the neural tube were dissected and 

 also sections were cut of the roof and floor. In order to get 

 shadows, specimens were tilted and rotated to secure all angles 

 and degrees of illumination. Kleinenberg's, Bouin's, and Gil- 

 son's killing fluids were used, and some of the embryos were 

 examined after faintly staining them with borax carmine and 

 Conklin's picro-haematoxylin. To supplement the study of 

 whole and dissected specimens, sections were cut in transverse, 

 frontal, and sagittal planes and stained with Ehrlich's acid 

 haematoxylin and Heidenhain's iron haematoxylin. 



No indication of anything that could be interpreted as seg- 

 mentation could be observed in the primitive streak, or before 

 the neural folds were clearly outlined as elevated ridges. At 

 this stage along the elevated margins of the medullary plate 

 certain lobulated irregularities are formed, giving a beaded 

 appearance to the crest, and these structures are present with 

 more or less uniformity in most embryos up to the closure of the 

 neural tube. They are, however, irregular in number in differ- 

 ent embryos and do not correspond in the two sides of the same 

 individual; their limits are often so obscure that they cannot be 

 determined with certainty, and the wide variation in their rela- 

 tive position makes it impossible to correlate them, either in the 

 two sides of the same embryo or in different embryos. They 

 differ greatly in size; often in the same embryo there are two or 

 three on one side while the corresponding region of the opposite 

 side will consist of a single lobe, or perhaps the edge will be 

 smooth, showing no lobulation. The variation in size, together 

 with the uncertain position and desultory arrangement suggest 

 strongly that this marginal lobulation is due entirely to differ- 

 ences in rate of cell proliferation at different points along the 

 rapidly expanding wall of tissue. 



As the neural crests increase in size, they rise rapidly and 

 begin to fold over toward the median plane. At this stage 

 especial care was exercised to detect any indication of segmenta- 

 tion in the medullary folds or in the plate between the folds. 



