MOEPHOLOGY OK TELEOSTEAN HEAD SKELETON. 509 



pituitary body (^j^.),ancl iufuiidibulum {inf.}, notwitbstandiug 

 tbe great distension of tbe head^ bave undergone no cbange 

 of position iu a longitudinal direction relatively to tbe 

 trabeculas and notocbord, proves tbat a close connection 

 exists between tbese and tbe floor of tbe brain. Tbe presence 

 of sbort, comparatively inelastic elements side by side with 

 a longer and more bigbly elastic and distensible one^ sucb as 

 tbe brain^ supplies us witb structural and niecbanical relations 

 analogous to those found in tbe Brequet's thermometer. In 

 the head of this sixth-day embryo^ under a cbange of external 

 conditions, the long, wealc, unrestrained dorsal portion of the 

 brain, from tbe optic nerve (11) to tbe line (*), distends more 

 tlian the sbort, strong, restrained ventral portion; and the 

 result is tbe well-marked bend. As development advances, 

 these opposite conditions are done away witb. The tissues 

 of tbe upper surface become stronger, and that portion of the 

 ventral surface supported by tbe trabeculte (fr.) and notocbord 

 {ch.) more extensive. Consequently tbe influence exerted by 

 tbe latter is no longer one of restraint, but of extension; 

 and therefore, at tbe time of hatching (fig. 58), the whole 

 bead straightens out from the curvature enforced upon it by 

 the zona (*"^). 



It would be digressing too far from tbe main line of the 

 present memoir to pursue tbis interesting topic further; suffice 

 it to point out that these facts, by showing the existence 

 of the necessary mechanical conditions, lend support to 

 Sewertzoff's hypothesis concerning the influence of tbe state 

 of development of one organ upon tbat of another (99, p. 319) ; 

 tbat, as shown by tbe observations of Dendy and Howes (99), 

 Howes and Swinnerton (01), upon tbe eggs of Sphenodon, 

 other pressures tban the mere passive resistance of tbe shell 

 exist ; and that possibly prematurely released embryos may 

 undergo other distortions, the study of which would throw 

 light upon the mechanics of development. 



It has been binted above that the first traces of the 

 head skeleton, other than the notocbord, appear during tbe 

 latter part of tbe sixth day. In my youngest embryo which 



VOL. 45, PART 4. — NEW SERIES. M M 



