24S ANIMAL MORPHOLOGY 



by an addition or subtraction of characteristics, but this is a com- 

 mon method. Very often we find one characteristic being replaced 

 by another. Thus in Lepidoptera one species may differ from another 

 in the replacement of red by yellow; or one earthworm will differ 

 from another by having the sexual openings in different segments. 

 We have no reason for thinking that these characteristics are 

 not integral entities as much as those distinguishing domestic 

 races. The modern morphologist, therefore, with the significance 

 of characteristics in mind, must appreciate that in enumerating 

 these characteristics he is enumerating the steps of evolution. 



The relations of morphology to embryology are so intimate that 

 the latter is commonly reckoned a subdivision of the former. Cer- 

 tainly the interpretation of the adult form depends on a knowledge 

 of its development. "In terms of the ancient riddle," says Bateson 

 (Nature, vol. LXX, p. 412), in his recent address as president of the 

 section of zoology in the British Association, " in terms of the ancient 

 riddle, we must reply that the owl's egg existed before the owl, and 

 if we hesitate about the owl we may be sure about the bantam." 

 The characteristics of the adult form are implicit in the fertilized 

 egg, and are determined by the Anlagen of the characteristics wrapped 

 up in that egg. We know now that upon the symmetry of the egg 

 and of the successive cleavages often, if not typically, the symmetry 

 of the adult form depends, 1 and that to the lack of symmetry of 

 cleavage in gasteropods their lack of symmetry is probably to be 

 referred. 2 In the successive cleavages definite organ-tracts are 

 marked off, 3 and still later the epidermal organs, such as hair, 

 feathers, and scales, the bearers of the more evident heredity 

 characteristics, are laid down in regular lines, radiating often from 

 single points or groups of cells, 4 thus simplifying the problem of 



1 H. E. Crampton, 1894, Reversal of Cleavage in a Sinistral Gasteropod, Annals 

 of New York Academy of Science, vni, 167-170. 



2 E. G. Conklin, The Embryology of Crepidula, Journal of Morphology, xnr, 

 1-210, April, 1897. 



3 Compare the results of H. E. Crampton, Experimental Studies in Gasteropod 

 Development, Archiv fur Enturickelungsmechanik, m, 1-19 (1896), in which the 

 removal of an early cleavage cell led to a corresponding defect in the larva. Even 

 more striking are the results of E. G. Conklin with ascidians, Organ-Forming Sub- 

 stances on the Eggs of Ascidians, Biological Bulletin, vin, 205-230, March, 1905, 

 who finds organ-tracts preformed in the uncleft egg. 



4 That metamerically repeated organs are laid down from definite bands of cells, 

 sometimes originating in "pole-cells," has long been known, through the studies of 

 Hatschek and of Whitman, Contribution to the History of the Germ-Layers in Clep- 

 Kine, Journal of Morphology, i, 108-179 (1887). Compare also the following: E. B. 

 Wilson, The Embryology of the Earthworm Journal of Morphology, in, 388-450 

 (1899); E. B. Wilson, The Origin of the Mesoblast- Bands in Annelida, Journal of 

 Morphology, iv, 205-219; W. M. Wheeler, Neuroblasts in the Arthopod Embryo, 

 Journal of Morphology, iv, 337-344. That multiple organs of other sorts are laid 

 down in linos appears for feathers from the work of Nitzsch,Pterylography(1867), 

 translated by P. L. Slater, Ray Society; and for hairs, J. C. H. de Meijere, Ueber 

 die Haare d(r Saugethiere, inbesonders uber ihre Anordnung, Morphologische Jahr- 

 buch, xxi, 312-424. 



