136 THE STRUCTURE OF MAN 



during its earliest development, passes through a monorhinal 

 stage, and probably more or less distinct traces of this can be 

 discovered in the embryos of all Vertebrates. 



From this median or unpaired olfactory plate, therefore, 

 which may be homologous with the anterior neuropore of 

 embryologists, and with the " olfactory organ " of Amphioxus, the 

 hypophysial tube arises, prior to the formation of the mouth, and, 

 growing down gradually, approaches the base of the brain till 

 it reaches the neighbourhood of the infundibulum. The epithelial 

 strand later separates off from the ectoderm, and finally to a great 



FIG. 86. MEDIAN LONGITUDINAL SECTION THROUGH THE HEAD OF A NEWLY - 



HATCHED LARVA OK THE SMALL LAMPREY (Petromyzon planeri). 



f.b., fore-brain ; m.b., mid-brain ; h.b., hind-brain ; ep., glandula pinealis ; ol., olfactory 

 organ ; hp. t hypophysis ; st. t buccal sac (stomodaeum) ; al., endodermal cavity 

 (mid-gut) ; ch., chorda dorsalis. 



extent degenerates, so that at last nothing remains of it but its 

 constricted, swollen end the glandular hypophysis of adult 

 anatomy. A somewhat similar arrangement is seen, as has 

 already been said, in Ammocoetes and certain tailed Amphibians. 



The facts appear to me strongly to confirm the view that > ( 

 the hypophysis corresponds with the primitive mouth (archi- ' 

 stoma) of the ancestors of the Vertebrata. 



The present vertebrate mouth (neostoma) is by some considered 

 to have arisen by the running together of a pair of branchial 

 clefts ; but this is by no means definitely proved. 



According to Scott, the close connection between the hypo- 

 physis and the oral invagination (stomodseum) of the higher 

 Vertebrates was developed secondarily in consequence of cephalic 

 flexure, due to the preponderating development of the fore-brain. 

 If so, the hypophysis had originally no relation either to the 

 mouth or the nose, but is to be regarded as an organ (? sensory), 



