Developmental history of primary segments of the vertebrate head, 409 



lu the dorsal view (Fig. 5) it will be observed that a lateral expansion 

 has taken place both caudad and cephalad to the deep constriction 

 if). The expansion involves segments 6 and 7 and occurs also in the 

 chick. The limits of the five anterior segments were marked by both 

 external and internal transverse grooves and these segments could 

 therefore be clearly counted. 



Since segments 2 and 3 are reciprocally wedge-shaped their re- 

 lative width in a dorsal view depends upon the optic plane in which 

 they are observed. The camera tracing of Fig. 5 is taken from a 

 median horizontal plane, and segments 2 and 3 are therefore equal 

 in size. 



As can be seen in the profile views, the embryos at this age arc 

 curved ventrally to conform to the spherical yolk upon which they 

 rest. Fig. 5 was sketched from two points of view. A camera 

 tracing was first made -of that portion of the head in front of the 

 cerebellum. The embryo was then rotated, till the line of vision 

 formed a right angle with the horizontal plane of the medulla and 

 another sketch made and attached to the former. Fig. 5 thus 

 represents the cephalic end, not curved, but straightened. 



Fig. 5 should be compared with Fig. 7, which shows the dorsal 

 view of a living embryo, 26 days old, as seen from one point of view 

 only. In the latter, the medulla is very much fore-shortened, and the 

 mid-brain much expanded laterally and presenting distinctly 2 seg- 

 ments {4 and 5). The primary fore-brain appears as a narrow tube 

 closely compressed between the large lateral eyes. The region in- 

 cluding the primary fore-brain and mid-brain, thus have become pear- 

 shaped with the pointed end turned cephalad. The 3 segments of 

 the fore-brain show in this view. The second (segment 2) is very 

 narrow in the dorsal region and widens ventrally (see side views), 

 while segment 3 is relatively broad in the dorsal region but very 

 narrow in the ventral zone. 



In dissected embryos one day older (Fig. 8) than the one just 

 described, the expansion of the cerebellum (segment 6) is very distinct. 

 The transverse external groove that marks its anterior limit, and thus 

 separates the cerebellum from the mid-brain, has become deep and 

 passes from the dorsal crest of the encephalon laterally and ventrally 

 towards the base of the brain where it disappears. The dorsal 

 groove if) that forms its posterior limit, still continues to be deeper 

 than the other transverse grooves of the medulla. The segments of 

 the latter have passed ventrad, a process that appeares to accompany 



