MORPHOLOGY OP TELEOSTEAN HEAD SKELETON. 507 



which have already taken on that highly vacuolated structure 

 so characteristic of advanced chordal tissue (PI. 30, fig. 25). 

 On the former day, whilst the medullary groove is still open 

 in front, the notochord runs parallel to the egg-shell for the 

 whole of its length ; on the latter a slight ventral and also 

 lateral displacement of the anterior or cranial end has taken 

 place. From this time onwards to the time of hatching, this 

 flexure increases no furthei-, but the chorda merely moulds 

 itself to the shape of the brain floor (fig. 58, ch/). 



Balfour (78, p. 210) has described a similar but much more 

 strongly marked flexure in Elasmobranchs, and has figured 

 it for embryos of the Torpedo, Pristiurus, and Scyllium. 

 In them it may take place through "an angle of 180V' and 

 " is not directly caused by the cranial flexure." Nor is 

 the slight flexure above described in the stickleback due 

 to this cause, for here, under normal conditions, I fail to 

 find any trace of a ci-anial flexure; the cause should rather 

 be sought for in this case in the rapid development and 

 consequent great increase in size of the brain, and in the 

 presence of a rigid zona radiata (fig. 56 **) which prevents 

 this organ from expanding dorsally. 



Under certain circumstances, however, the cranial notochord 

 may exhibit an extremely well-marked ventral flexure. In 

 figs. 56 and 57 are represented, diagraramatically, medium 

 longitudinal sections through embrjos of exactly the same 

 age, viz. three days before the time for hatching; but whilst 

 fig. 56 represents one killed within the egg, the other is 

 taken from one which was killed after being artificially 

 released. In the former all the parts of the head were 

 packed closely together, and the head itself flattened out 

 upon the yolk (figs. 54 and 56) ; in the latter (figs. 55 and 57) 

 it is no longer flattened but distended, and has undergone a 

 well-marked ventral flexure, chiefly through the region of the 

 hind brain (bJi.). The roof of the mid-brain (6. w.) con- 

 sequently comes to be the most anterior point of the body, 

 the axis of which is indicated by the arrow. This flexure is 

 still more conspicuously shown by the notochord, which 



