280 ANIMAL PHYSIOLOGY. [LESSON 86. 



concentrated ; hence the absorption of ganglions in the abdomen, 

 where they are no longer wanted, and the deposition of increased 

 nervous matter in the thoracic ganglions, where it is imperatively 

 needed. 



These facts are beautifully illustrated in the disparity of size and 

 number of the nervous centres (ganglions) of the active Slugs and 

 Snails, as compared with the passive bivalve-mollusca. 



It has been remarked, with great propriety, that, in the progress 

 of his development, man passes successively through all those phases 

 of existence at which the remainder of the animal kingdom, respec- 

 tively, remain permanent. 



He has never been, in reality, an Animalcule, an Insect, a Mol- 

 lusc, Fish, or Reptile, but he has passed through the conditions at 

 which they remain stationary. 



It has been shown that, in the development of the human brain 

 and spinal chord, two nervous chords, unprovided with ganglia, first 

 exist. 



The next stage exhibits a single ganglion, and at this moment 

 the human embryo is on the same plane with those animals in whom 

 a sole ganglion becomes permanent ; as the ganglia increase slowly 

 in number, so the embryo attains a higher rank in the scale of being. 



But it is equally true that, prior to the production of even a 

 nervous chord, a still lower type was indicated, for, although the 

 statement is somewhat humiliating to the " lord of creation," we yet 

 commence our career in a form far less animal than vegetable. At 

 one period of the development of the ovum, a single cell exists ; 

 presently it divides into two ; these sub-divide into four, and this 

 process continues until the yelk consists entirely of a congeries of 

 cells. 



Now this is precisely the plan upon which, as we have seen, 

 plants owe their being, and in both kingdoms the primary cells are 

 employed to organize the tubular tissues spiral and lactiferous ves- 

 sels in plants, nutrimental organs and blood-vessels in animals. 



The probability is that man possesses a stomach, not only before 

 the advent of any portion of the nervous system, but prior to the 

 formation of an intestinal canal, thereby reducing him to the level 

 of the polypes. When the intestine is superadded, he has attained 

 a higher grade of development. 



Again, the stomach is completed, like that 'of the polype, with 

 only one aperture ; when other cells are added, to form a rudimen- 

 tary and short intestine, the septum (partition) becomes absorbed. 



