342 HORACE W. STUNKAED 



In some specimens other similar divisions appear posterior to 

 these, but they are less distinct and gradually fade out pos- 

 teriorly so that their number could not be determined. Nor- 

 mally, the transverse grooves first appear at the lateral edges of 

 the plate and extend toward the neural groove. Frequently one 

 appears on either side before the others and since these first 

 ones are at a corresponding level, their fusion forms a furrow 

 which I regard as the transverse cephalic groove of Griggs. 

 There is considerable irregularity in the formation of the grooves, 

 however; often those of the two sides are formed at different 

 levels and do not meet at the median line. The areas between 

 the grooves are then irregular in size and shape; frequently they 

 are almost triangular as the transverse grooves converge or 

 meet, either at the neural groove or at the lateral edge of the 

 plate. The transverse grooves in the two sides of the neural 

 plate of the same embryo do not regularly correspond, and this 

 lack of correspondence is manifest in the figures of Griggs and 

 other authors. Only in the occasional and unusual specimen is 

 there present the regular arrangement described by Griggs. The 

 neural folds close rapidly and it is possible to observe the changes 

 that take place during the process. It is a significant fact 

 that the grooves do not always retain precisely their original 

 aspect during the closing process. Some of the grooves shift 

 slightly or fade out entirely and other grooves appear in different 

 positions. 



Divisions of the neural plate caused by the transverse grooves 

 could not be clearly or satisfactorily demonstrated in sections, 

 but such study shows that, with the appearance of the transverse 

 grooves, the mesoderm has developed to a stage where it is 

 assuming a segmented condition, and I regard the formation 

 of the transverse grooves as due to the formation of the meso- 

 dermal somites. I am convinced that certain of the trans- 

 verse grooves coincide with the divisions between somites, and 

 I am inclined to beheve that it is true of most if not all of 

 the transverse grooves. It is possible, however, that grooves 

 are also due to associated mechanical factors, pressure produced 

 by the multiplying cells and the infolding of the neural crests. 



