COMPARISON OF ARTHROPOD AND VERTEBRATE EMBRYOS. 33 



finally, there is present in all arthropods that have been carefully studied in regard 

 to this organ, a median, subneural cord agreeing in position, development, and in 

 some cases in function, with the notochord of vertebrates. (Figs. 221-231.) 



It is evident, therefore, that the resemblance in form and general appearance 

 between the ostracoderms and the marine arachnids is not a fanciful one, to be 

 classed as a meaningless coincidence, or as due to mimicry, to parallelism, or to 

 a particular mode of life. The resemblance is real, and pervades the whole 

 organism, and can be satisfactorily explained only on the assumption that there 

 is a close genetic relationship between the two classes. 



III. COMPARISON OF ARTHROPOD AND OF VERTEBRATE EMBRYOS. 



A comparison of adult arachnids with adult vertebrates helps us to see the 

 morphological relations that exist between the two types, but it cannot tell us how 

 one arose from the other. That is the function of comparative embryology, for 

 the rise of one great class from another takes place during the malleable embry- 

 onic periods, when transitional stages are created by a slow yielding to the impact 

 of successive readjustments between organs developing under unequal and un- 

 stable conditions. 



Hence the supreme test of any broad theory of phylogeny is its ability to pre- 

 sent an unbroken series of embryonic stages, naturally or inevitably leading from 

 one type to the other, and to point out the efficient causes for them. This embryonic 

 series should include, at the proper period, the characteristic anatomical structures 

 of both types. The established direction of growth shown by various systems of 

 organs, and the general conditions that control growth in the lower type, should 

 persist in the higher, supplying a past cause for the creation of the fundamental 

 features of the new type, and a present one for those now appearing in it. There 

 should be no changes demanded that necessitate the sudden destruction of old 

 organs, or the abrupt creation of new ones; that interrupt the continuity of in- 

 dividual life, or that break, or entangle, the necessary morphological relations of 

 one organ to another. This dual embryonic series should run parallel with, 

 and should supplement and elucidate the series of adult forms left along the trail 

 made by the two types in their slow process of evolution. 



I have made a series of models that show how the embryos of vertebrates and 

 arachnids conform to these requirements. (Figs. 24-34.) They are intended 

 to illustrate the principal stages in the development of a primitive vertebrate 

 supposed to be descended from arachnids. The series begins as an arachnid 

 embryo and leads, without any greater changes than are found in the develop- 

 ment of the higher animals, through the typical embryonic stages of forms like 

 Limulus and scorpion, into a vertebrate embryo of the fish-like or amphibian type. 



The series shows us that the early stages of vertebrate embryos, in all essen- 

 tial respects, run parallel to, or are identical with, those of arachnids; and that the 

 same morphogenic forces which created the cephalothorax of arachnids find their 

 full expression in the head of vertebrates. It shows that both embryos begin 



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