INCREASING VOLUME OF THE YOLK SPHERE AND BRAIN. 461 



importance in controlling the mode of life, the method of growth and the direc- 

 tion and progress of evolution in these phyla. 



In the craniates, the characteristic chitenous exoskeleton of the arthropods 

 is doomed to extinction, owing to conditions created by its own mode of growth. 

 It has been shown, for example, that in Limulus a peculiar and exceptionally 

 vigorous method of growth in the skeletogenous tissues gives rise to a system 

 of subdermal, interlocking trabeculae that probibits the subsequent periodic 

 removal of the exoskeleton. A condition is thus produced that compels the per- 

 manent retention of the chitenous products within the tissues of the animal, and 

 which initiates the formation of a new type of exoskeleton that is largely sub- 

 dermal, cellular, and fragmented. This permits a new mode of growth for the 

 animal as a whole, differing from the old in much the same way that the growth 

 of an endogenous stem differs from that of an exogenous one; and it ultimately 

 liberated the arthropod stock from the bondage of an increasingly restrictive, 

 burdensome, and menacing armor. This new type of skeleton itself practically 

 disappears in the higher vertebrates, giving place to the new framework of carti- 

 lages and bone that constitute the internal skeleton. 



D. The Increasing Volume of the Yolk Sphere as a Creative Factor. 

 The local retardation of growth caused by the presence of yolk in the developing 

 ovum has long been recognized by embryologists, but they have not recognized 

 the form controlling conditions created by apical growth on a spherical surface. 



The gradual increase in the size of the yolk sphere throughout the arthropod 

 and lower stages of the vertebrate stock creates a new set of conditions that greatly 

 modifies the process of development; for the growth of an embryonic metamere 

 over the surface of a yolk sphere is a different problem from the growth of a new 

 metamere added to the apex of a mature animal. One spreads film-like in mer- 

 cator projection over an approximately plane surface, the other grows as a solid 

 body round a central point. When the size of the yolk sphere is increased, the 

 number of metameres so affected is increased, and the effect on each metamere 

 will depend on its location in the series, that is, whether it lies at the head or tail 

 end, whether it has to grow round the equator, or round the poles of the sphere. 

 In this way the size of the yolk sphere controls the structure and mode of growth of 

 the heart, the belly navel, and germ wall, and has created the phenomenon of 

 concrescence. 



Thus, owing to the difference in the location of metameres on a yolk sphere 

 of variable dimensions, inevitable differences arise in the conditions under which 

 these metameres are compelled to grow, and these differences are increased, or 

 exaggerated, with the increasing volume of the yolk sphere. These differences 

 in conditions coincide to a large extent with the morphological and physiological 

 differences that characterize the corresponding regions of the body, and may be 

 assumed to be the causes that have brought them about. 



E. The Increasing Volume of the Brain as a Creative Factor. A con- 

 spicuous feature in the evolution of the arthropods is the steady increase in the 



