254 



THE OLD MOUTH AND THE NEW. 



given case, by the shape and volume of the yolk sphere, and by the amount of 

 " cephalization " that has taken place. 



"Cephalization" takes place according to a definite law of growth which 

 apphes to all segmented animals. According to this law, the lateral members of 

 the anterior metameres tend to atrophy in proportion to their relative distance from 

 the median line and their nearness to the anterior end.- In other words, there is 

 a steadily progressive tendency to eliminate by degeneration, the lateral members 

 of the more anterior metameres, and to enlarge and specialize the median ones.-^ 



Thus in the higher arthropods there are no fully developed appendages or 



c 



I 



T) 



Fig, 156. — Diagrams to illustrate the method of bilateral apical growth. A, Hypothetical symmetrical 

 checker board arrangement of unlike parts in mercator projection, seen from the neural surface; The serial 

 equality (homology) , of the elements is supposed to be perfect. B and C, Approximate method of producing 

 such a field by a combination of apical and bilateral division of units. D, The same, seen as a cylindrical object 

 from the side. E, The same, in cross-section, composed of four, unlike concentric superimposed layers. 



other somatic organs lateral to the procephalic lobes; and no organs in the lateral 

 plate region of the first nine or ten metameres. The scanty, or degenerating, 

 tissues that form where the lateral organs should be, are gradually crowded by the 

 growth of the more vigorous posterior ones into an oval area on the heemal surface 

 of the embryo, just behind the forebrain, where the degenerating haemal blasto- 

 derm sinks into the yolk and is absorbed. (Fig. 135.) 



In insects, the ruptured embryonic membranes play an important part in this 

 process; they complicate it, but do not alter its essential nature. In many arth- 

 ropod embryos, the dorsal organ becomes a formal invagination into the yolk, 

 with well-defined epithelial walls, or it may consist merely of a great cloud of in- 

 growing cells. In both cases, the deeper cells are the first to dissolve in the yolk 

 mass that will later be enclosed within the midgut. 



A " dorsal organ " of some such nature as this occurs in all classes of arthropods. 



Not to be confused with median fusion and degeneration. See p. 277. 



