Developmental history of primary segments of the vertebrate head. 



405 



certainty the anterior limit of the latter. Caudad to this depression 

 there are 5 distinct segments that constitute the medulla (7 — U). 



The auditory vesicle lies opposite segment 10. Caudad to this 

 vesicle and at a distance equal to two segments of the medulla, or 

 approximately to the width of three myelomeres, is the first body 

 somite. I was unable to observe, at this stage, any segmentation 

 in the heavy mesoblastic pad that occupies the space between this 

 somite and the auditory vesicle. 



Cephalad to the deep groove (f), the neural axis is divided into 

 6 segments by the presence of five transverse dorsal grooves, that 

 appear fainter but otherwise resemble the grooves that divide the 

 medulla into segments. The one between segments 3 and 4 (c) is 

 deeper than the other four, and as the posterior commissure later 

 appears at this point, we may speak of this groove as the dividing 

 line between the primary fore-brain and mid-brain. The former is 

 therefore made up of 3 segments {1 — 5); the mid -brain and 

 cerebellum likewise of 3 segments {4 — 6). As will appear later, 

 segment 6 develops into the cerebellum and segments 4 and 5 there- 

 fore represent the mid-brain. In older embryonic stages these en- 

 cephalic divisions become very distinct and can be traced with ac- 

 curacy in both living and dissected as well as divided specimens. 



Fig. B. 



In a living embryo, 2 days older (sketch B), the head fold is 

 a little deeper, and the optic vesicle, which now presents a cavity, is 

 folded back upon itself to form the optic cup. The lens is completely 

 constructed from the epiblast. The auditory vesicle lies a little farther 

 caudad and partly covers segments 10 and 11. The 11 encephalic 

 segments are present as described in sketch A, but the dividing 

 grooves are deeper and can be traced a short distance down the sides 



