34 



OUTLINE OF THE ARACHNID THEORY. 



pc.mx 

 . mx 



their growth from the same surface of the egg and spread over the yolk in the 

 same directions, enclosing the opposite side in the same manner; that there has 

 been no transfer of the nerve cord from one surface to the other, as claimed by 

 Gaskell, and that the medullary plates in both types are homologous, and not, as 

 claimed by C. L. Herrick, one dorsal, the other ventral. 



Form Controlling Factors in the Early Stages. Apical growth and the 

 volume and composition of the yolk sphere are impor- 

 tant factors in the development of the embryo, because 

 the physical and chemical composition of the yolk 

 sphere controls the rate of radial growth, while the 

 circumference of the yolk sphere, and the ratio 

 between the rate of apical and bilateral growth, deter- 

 mines the relative time and place at which certain 

 organs arise, and the physical conditions under which 

 they develop. Owing to the relatively large volume 

 of the yolk sphere in the arachnids, neither the ccelente- 

 rate nor trochosphere stages can assume the form of 

 the ancestral, free swimming animal, i.e., a nearly 

 spherical body growing in each of three dimensions at 

 about an equal rate, for they are reproduced in the 

 arachnid egg under totally different conditions. They 

 appear at a time when cell growth is beginning on the 

 outer surface of a relatively large sphere of inert ma- 

 terial, and the various organs must be mapped out in 

 one plane, like a Mercator projection of the earth's 

 surface. Moreover at these early stages, the develop- 



FIG. 23 . -Diagram illustrating ment of the dee P er lying organs is delayed, owing to 



a hypothetical, transitional condi- the impenetrability of the yolk and the lack of respira- 

 tion, between the embryo of a 



marine arachnid and that of a tory facilities. 



primitive vertebrate It shows the Th jj th j . t fe expressed j n 



convergence of procephalon, appen- J 



dicuiar arches, and mesodermic the form of a film in which the rate of growth, in the 



lateral plates around the dorsal , , . . . .,.,,,. 



organ to form the cephalic navel, tnr ee dimensions, is very unequal. This film increases 

 or the aniage of the h^mostoma. j n i en gth by a process of apical growth, which takes 



The uncovered yolk, that is sur- J 



rounded by the concrescing germ place at one end only; in breadth by bilateral growth, 



walls and cardiomeres, constitutes i , r i i ^ ^ ^i rni i 



the beiiy navel, s. TV. anc * m thickness by radial growth. These early con- 



ditions are identical for all segmented animals, and it 



is only necessary to fix the location of the growing apex, and the direction of 

 bilateral growth, in order to fix, beyond question, the identity of the head and 

 tail ends and the neural and haemal surfaces. 



The Gastrula, Coelenterate, or Trochosphere Stage. The first stage 

 after cleavage (Fig. 24, A), shows the primitive cumulus, the primary center for the 

 origin of the germ layers. The central depression, gst, marks an area of inward 

 proliferation which gives rise to the endoderm, yolk cells, and procephalic meso- 



