Developmental history of primary segments of the vertebrate head. 407 



specimen lies nearly opposite segment 10, its posterior margin over- 

 lapping a part of segment 11. The lateral expansion that was 

 mentioned in describing 18 day embryos (sketch B) has gradually 

 increased and involves the dorsal portion of segments 6 and 7. It 

 should be remembered that only the anterior half of this expansion, or 

 segment 6, represents the cerebellum. In profile view, Fig. 1, it will 

 be seen that ventrally, this expansion is slightly directed cephalad. 



The transverse constrictions that mark the limits of the five ceph- 

 alad segments are less distinct than those of the medulla, but like 

 the latter pass ventrally around the encephalon (Fig. 1). Segment 1 

 is elliptical and forms the front end of the neural axis. Segment 2 

 has the form of a wedge. Its broad end is turned ventrad and slightly 

 expanded to form the first traces of the infundibulum (inf). Segment 3 

 has also the form of a wedge but in this case the broad end is 

 turned dorsad. These 3 segments (i, 2 and 5) develop into the 

 primary fore-brain as will appear more clearly in later stages. The 

 2 segments of the mid-brain (4 and 5) are also well defined by 

 the presence of external, transverse, segmental grooves. 



The presence of these transverse grooves in the fore-part of the 

 brain gives a clear picture of segments or joints. As these grooves at 

 this stage of development encircle the neural axis the segments em- 

 brace both dorsal and ventral zones. As will be observed later the 

 appearance of the inner walls of the fore- and mid-brains in the 

 specimens is worthy of especial notice. The lines separating the seg- 

 ments are definite and the number of segments can be counted on 

 both the inner and the outer aspect of the encephalon. In both 

 cases, there are 5 segments in this region, 3 in the primary fore- 

 brain and 2 in the mid-brain. The transformation of these segments 

 is interesting and if closely followed clears up some obscure points: 



1) The most anterior segment is that of the olfactory, or segment 1. 



2) The optic nerve and the infundibulum come from segment 2; 

 the pineal outgrowth from the roof of segment 3. 



Fig. 3, Plate 28, shows the left profile view of the encephalon of 

 a dissected embryo 22 days old, with 34 somites. The lateral ex- 

 pansion of segments 6 and 7 is no longer restricted to the dorsal 

 region but affects the ventral portion of these segments. The ventral 

 growth of the second segment to form the infundibulum is much ad- 

 vanced and has its distal end directed caudad. In the ventral and 

 anterior margin of this segment is a small circular depression (opt. n) 

 which marks the union of the optic stalk with the encephalon. 



Zool. Jahrb. XIIl. Abth. f. Morph. cyrj 



