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 Amphiouus, 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 
<f-—- . 
Fic. 86.—MEpDIAN LONGITUDINAL SECTION THROUGH THE HEAD OF A NEWLY- 
HATCHED LARVA OF THE SMALL LAMPREY (Petromyzon planeri). 
f.6., fore-brain ; m.6., mid-brain ; h.b., hind-brain ; ep., glandula pinealis ; o/., olfactory 
organ ; hp., hypophysis ; st., buccal sac (stomodeum); 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 Ammoccetes 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 (stomodeum) 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), 
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