Integrative Systems 227 



carbohydrates, and affects blood-pressure. It is definitely known that 

 it stimulates contraction of the muscles of the mammalian uterus. 



Pineal Body 



In the description of the diencephalon (see p. 145) were mentioned 

 two median dorsal outgrowths, the epiphyses or parietal and pineal 

 structures (Fig. 138). Among fishes, amphibians, and reptiles are 

 animals in which one or the other, rarely both, of these epiphyses may 

 develop into a small, eyelike organ. Birds and mammals possess nothing 

 resembling a median eye, but the thin roof of the diencephalon does 

 give rise to a structure which appears in the adult as a small, solid body 

 projecting outward just anterior to the optic lobes (corpora quad- 

 rigemina of mammals: Fig. 140, p). In position and manner of origin, 

 it seems to correspond to the more posterior or pineal epiphysis of 

 other vertebrates. Known as the "pineal body," it is usually included 

 in the list of endocrinal glands. Histologically its structure is indiffer- 

 ent. The main reason for regarding it as a gland is that it seems less 

 likely that it could be anything else. Secretion is a function which is 

 not necessarily associated with conspicuous differentiation of cellular 

 structure. In the human brain, the pineal organ is deeply buried 

 beneath the bulky overhanging masses of the cerebral hemispheres and 

 cerebellum (Figs. 190, 139). The "body" being thus concealed in the 

 innermost recesses of the brain, the seventeenth-century philosopher, 

 Descartes, conceived it to be the seat of the soul. The considerable 

 amount of experimental investigation to which it has been subjected 

 has not proved that it is anything else. At most, there is some incon- 

 clusive evidence that it plays some part in the control of development 

 and growth. It remains a mystery. 



The Vertebrate "Pattern'"' — Basic but Flexible 



The foregoing survey of the "basic features" of vertebrates makes 

 it apparent that certain of their features are to be seen clearly only 

 in embryonic stages or, indeed, may not be recognized at all in the 

 adult. A study of the adult mammal would fail to reveal any pharyn- 

 geal clefts or notochord. In view of the fact that the very early em- 

 bryonic stages of all animals, invertebrate and vertebrate, are strik- 

 ingly similar, it may be objected that recourse to the embryo in the 

 search for common features distinctive of vertebrates is not valid. 



It is true that eggs of all animals are essentially alike in so far as 

 they are all minute bodies of protoplasm (plus more or less food mate- 

 rial), each organized as a single cell. The diverse organs which they are 

 capable of producing are not visibly present, as such, in the egg. Cer- 

 tain early stages — the one-layered blastula and the two-layered gas- 



