Developmental history of primary segments of the vertebrate head. 415 



15 hours and 5 days. Most of the embryos removed were between 

 20 and 40 hours. 



Beginning observations on embryos 45 hours old, in which the 

 segments of the medulla are unmistakable, and proceeding carefully 

 to earlier stages, I have traced these segments in unbroken continuity 

 back to embryos with one somite. In older stages they have been 

 traced forward to the fourth day of incubation after which they 

 disappear. 



The age of embryos has been determined by reference to Duval's 

 Atlas d'Embryologie. 



a) Embryos with open Neural Grooves (1—6 Somites), 

 Ages 21 — 25 hours. 



When the cephalic mesoderm is dissected away from the neural 

 grooves of embryos with ö'/g somites and the specimens studied by 

 reflected light, 11 transverse neural constrictions (Plate 29, Fig. 22 

 a— Ä;), are to be seen which divide the neural axis into ll^/g nearly 

 equal segments. Figs. 22 and 23 represent respectively dorsal and 

 lateral views of the same encephalou of an embryo 24 hours old, with 

 5^2 somites, while Fig. 24 represents the encephalon of the same 

 embryo after it has been divided and the inner surface of the left 

 half exposed to view. In the dorsal and lateral views (Figs. 22, 23), 

 the 11 transverse constrictions or grooves can be traced ventrally 

 around the whole neural axis. The neural segments, therefore, in- 

 clude ventral elements as well as dorsal. If Figs. 22 and 23 be 

 examined more closely it will be observed that the 3rd and 5th 

 grooves (c and e) are deeper than any of the others. In older 

 specimens it is found that these grooves form not only the posterior 

 borders of segments 3 and 5 but also mark the anterior limits 

 respectively of the mid-brain and the hind-brain. While the conven- 

 tional division of the encephalon into primary fore-brain, mid-brain 

 and hind-brain has a historical value it is to be observed that an 

 earlier segmentation is incorporated into this division as follows: In 

 the fore-brain, 3 primary segments are included (Figs. 22 and 23 

 1 — 5); in the mid-brain, 2 (4 and 5) and in the hind-brain, 6 

 (6—11)^ or 6^2 if the portion of segment 12 that lies in front of the 

 first somite is added to the latter. 



If we now direct our attention to Fig. 24, which represents the 

 encephalon of the same embryo after it has been divided and the 

 inner surface of the left half exposed, we observe, 1) that along the 



