INTERPRETATION OF THE EARLY STAGES. 221 



mass, over which the cells spread in all directions from an initial center that always 

 represents the beginning of the oral or neural surface. Around this center, the 

 various organs are arranged in a definite order, from the center outward. 



Embryonic growth on a yolk sphere, therefore, always begins near, or 

 centers in, the primitive oral region and spreads from it toward the aboral surface. 



If there is but little yolk present, the aboral surface may be formed during 

 cleavage, and at practically the same time as the oral surface. But in proportion 

 as the yolk increases in volume the formation of the aboral surface is delayed, 

 because it can only be completed by the growth of the margins of the germinal 

 disc around the yolk. 



Thus growth on the oral and growth on the aboral surface of the embryo are 

 totally distinct processes, and always take place under different conditions and 

 in opposite directions. On one side it is centrifugal, on the other centripetal. 

 The uncovered yolk mass, or yolk navel, always lies on the opposite side of the 

 egg from the blastopore and the germinal axis. 



The subject of apical and bilateral growth, or of radial and bilateral symmetry 

 has more than a purely academic interest for us, because the interpretation of bilat- 

 eral animals in terms of radiate ones, is the key to the problems of germ layers, 

 gastrulation, and concrescence, throughout the entire series of segmented animals. 

 The conditions creating bilateral symmetry and metamerism are so fundamental, 

 that it is hardly conceivable they could be otherwise than they are. There is no 

 reason whatever to doubt that the fundamental relations of the nervous axis, 

 blastopore, primitive mouth, and yolk sphere, and the main axes of differential 

 growth are the same in coelenterates and in all bilateral acrogenous animals. 



The axes of growth and of differentiation are the most important means of 

 orientation, and should be carefully considered in all attempts to compare one 

 great group of animals with another. 



Gaskell, Herrick, and others fail to recognize these fundamental relations. 

 Gaskell maintains that the " ventral" or neural side of an arthropod is the 

 same as the haemal or " ventral" side of a vertebrate, and that the vertebrate nerve 

 cords represent those of an arthropod, transferred from the "ventral" side of the 

 one to the " dorsal" side of the other. There are only two possible ways in which 

 such a transfer could take place. One way would be for them to migrate over 

 the surface of the yolk, right and left, uniting on the opposite side. In this case, 

 among other equally obvious difficulties, the original lateral margins of the cords 

 would be united in the median line, all the ectodermic territory, originally covered 

 by, and giving rise to the peripheral nerves and sense organs, would be extinguished; 

 and the outgrowing peripheral nerves would be directed into the canalis centralis, 

 with no visible means of escape. The second possible way would be for the cords 

 to migrate bodily through the yolk, in which case they would reach the opposite 

 side inside out, or upside down; that is, with the proliferating nuclear surface on 

 the deeper face of the cords, instead of the outermost surface as it actually is. In 

 order to meet the demands of his theory, Gaskell turns the arthropod embryo 



