PRIMARY NETJROMERES AND HEAD SEGMENTATION 341 



neural groove forms without the previous appearance of separate 

 depressions. Griggs stated that it is sometimes impossible to 

 distinguish between the anterior depression, posterior depression, 

 and neural groove. The depressions described by Griggs appear 

 in the same position as the neural groove, and I see no good 

 reason for considering them as anything other than stages in 

 the formation of the neural groove itself. The anterior end of the 

 neural groove is marked by a depression, the anterior pit of 

 Griggs, which becomes deeper, extends anteriorly, and, with only 

 slight variations in the process of development, becomes the 

 infundibulum. Tn these early stages it is sometimes possible to 

 distinguish in the neural groove faint alternating lighter and 

 darker areas, which in a few specimens suggest a segmental 

 condition, but observation of a large number of embryos shows 

 such variation and irregularity as to preclude such an interpre- 

 tation. 



Lateral longitudinal depressions at the sides of the neural 

 plate could not be distinguished before the appearance of the 

 neural crests. With the thickening of the ectoderm to form the 

 crests, these structures are slightly elevated and a lateral 

 linear* depression is visible, not only on the median, but often 

 also on the lateral side of the neural crest. The neural ridges 

 increase in size and length, growing anteriorly, posteriorly, and 

 dorsally until they become continuous in front of and behind 

 the neural plate. As the neural crests grow dorsally, the anterior 

 part of the neural fold rises prominently, and the embryo has the 

 appearance shown in figure 1. At this stage the blastopore has 

 closed to a narrow sHt and the neural groove extends from the 

 blastopore almost to the anterior part of the neural fold. On 

 either side, just caudal to and within this anterior part of the 

 fold, there is visible occasionally a small depressed circular or 

 oval deeply pigmented area. These depressions were described 

 by Eycleshymer, and, according to him, are the initial stages of 

 the paired eyes. 



In many of the embryos a few (usually three to five) faint 

 transverse grooves appear in the anterior part of the medullary 

 plate, but they are not constant in number or regular in position. 



