506 BERTRAM G. SMITH 



grooves are equidistant, and so distinct that they may readily 

 be seen with the naked eye; since they regularly appear in the 

 same order and position in different embryos, and persist through- 

 out the further history of the open neural plate, they serve as 

 trustworthy landmarks during the following stages. In the 

 figures they are numbered with Roman numerals. By following 

 their history through later stages they have been traced to the 

 region of the medulla oblongata of the adult brain ; consequently, 

 at least all that portion of the neural plate in front of Groove III 

 belongs to the cephalic plate. 



The early segmentation of the cephalic plate in front of Groove 

 I will now be considered. There first appears a transverse groove 

 dividing this region into two portions of which the posterior is 

 slightly the smaller (figs. 155 and 156) ; the anterior of these 

 areas is then crossed by two more grooves (figs. 157 and 158), 

 while the posterior area is for the present doubtfully segmented. 

 The smaller transverse grooves occurring in various parts of the 

 cephalic plate are irregular in position and probably are of no 

 segmental value; most of them disappear in later stages. Those 

 grooves in front of Groove I which are regarded as of metameric 

 value are numbered with Arabic numerals, consecutively and 

 without regard to the order of appearance. 



The question naturally arises whether these early transverse 

 divisions of the cephalic plate are neural in origin or secondarily 

 produced by the segmentation of the underlying mesoderm. 

 This question has not yet been thoroughly investigated by the 

 study of sections, but the results of a preliminary examination 

 favor the idea that in front of Groove I at least, they are primarily 

 neural structures ; the mesoderm, particularly in front of Groove 

 I, is at this time quite thin as compared with the neural plate, 

 and hardly capable of producing the modifications of the latter 

 layer. 



Since Grooves I, II, III, etc. (see also Stage 14) are produced 

 in regular order from before backward there is ground for sus- 

 picion that they are intimately connected with the formation of 

 the mesoblastic somites. In view of the fact that the segmenta- 

 tion of the region immediately in front of Groove I is late in 



