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G. R. DE BEEll 



surface of the head so as to leave the hypophysis depression 

 as a tube which becomes deeper by the lengthening of its walls. 

 The beginning of this process is shown in Text-fig. 3, and it 

 is more accentuated in Text-figs, 4 and 5. 



The apex of the V, i. e. the limit of the depression, does not 

 move farther back relatively to the rest of the head. It never 



Petromyzon plane ri. Embryo fourteen days old, 

 beginning of expansion of the upper lip. 



quite reaches the optic chiasma. Meanwhile the soHd ingrowth 

 has extended until it almost reaches the tip of the notochord. 

 To verify this I have resorted to measurements, though I do not 

 set much store by them owing to the difficulty of measuring 

 \\-ith accuracy and of selecting standard measuring lines since 

 no points can be assumed to be fixed, 



A. Distance from apex of depression to notochord. 



B. Distance from apex of depression to extremity of upper hp. 



