224 GRAND STRATEGY OF EVOLUTION 



FIGURE 8. THE SAME AS FIGURE 7, SEEN IN MEDIAN SECTION. The chief 

 events indicated are: (i) the closing of the old gateway to the gut and the 

 opening of a new one in the region of the cephalic navel, c.n.; (2) the in- 

 folding of respiratory appendages of the phyllopod to form arachnid gill 

 sacs, or lung-books; their rupture into the alimentary canal in fishes, 

 their subsequent atrophy and finally the substitution of lungs to take their 

 place; (3) the origin of the chief ductless glands, the pituitary body from 

 the remnants of the old oesophagus, and from an ancient excretory organ, 

 pf, pf; the pineal gland from the median eye, p.e., and the thyroid gland, 

 th, from a segmental outgrowth of the alimentary canal ; (4) the meta- 

 morphosis of the primitive heart tube into the four chambered heart; (5) 

 the metamorphosis of three pairs of arthropod appendages into the man- 

 dibles, maxillae, and pre-maxillae of vertebrates; (6) the rise of the 

 notochord, nch, neural arches, n.a,, and cartilagenous cranium, cr, in the 

 arachnids, and their further growth into the vertebrate column and skull 

 of vertebrates; (7) the rise of excretory and genital organs and their ducts 

 in the anterior end of the body and their successive transfer toward 

 the posterior end; (8) the establishment of the main divisions of the brain 

 in the phyllopods and their persistent growth into the characteristic sub- 

 divisions of the brain in the higher vertebrates. 



