HISTORICAL SKETCH. 



The resemblance between vertebrates and arthropods first attracted my 

 attention in 1884. In my paper on the development of the phryganids, it was 

 stated, page 594, that a wonderful analogy, if not homology, exists between the 

 structure and mode of growth of the medullary plate, the neural and gastrular in- 

 vaginations, and the neurenteric canal of insects and the corresponding structures 

 in vertebrates. 



Three years later, a resemblance between the minute structure of the 

 compound eyes of arthropods and the retina of vertebrates was recognized. 



In 1888, it was shown that the invagination of the procephalic lobes, supposed 

 by writers of that period to give rise to a two-layered compound eye, in reality 

 gave rise to the optic ganglion only, while the eye itself consisted of a single layer. 

 Further study of the developing brain and eyes of Acilius, Buthus, and Limulus, 

 showed that in many arthropods the procephalic lobes underwent a complex 

 process of invagination, accompanied by the overgrowth of a neural crest, or 

 palial fold, the result being the formation of a vesicular forebrain, and the transfer 

 of the ocelli, located on the outer margin of the lobes, to the end of a tubular or 

 epiphysial outgrowth of the membranous roof of the forebrain vesicle. 



Here were revealed, for the first time, all the steps in the transformation 

 of an invertebrate type of eye into the type of eye so characteristic of ver- 

 tebrates. This apparently simple fact was in reality the result of very complex 

 conditions, and it seemed incredible that they could be duplicated except in 

 animals belonging to the same stock. 



These discoveries, therefore, appeared so profoundly significant that I deter- 

 mined to follow the clue to the end, to see whether further analysis of the eyes, the 

 brain, and other systems of organs would not confirm the obvious conclusion to be 

 drawn from them. The results proved to be so surprisingly in accord with them, 

 that in the following year, 1889, a definite theory was formulated, and a prelim- 

 inary sketch, or outline, of it was published under the title "On the Origin of 

 Vertebrates from Arachnids." 



This theory has formed the basis of all my subsequent work, and as far as it 

 went, is practically the same as the one presented here. In that paper it was 

 maintained that the vertebrates are descended from the arachnid division of the 

 arthropods, in which were included the typical arachnids, the trilobites, and 

 merostomes. The ostracoderms were regarded as a separate class, uniting 

 the arachnids with the true vertebrates. Limulus and the scorpion were the 

 types most carefully studied, because they were the nearest and most available 



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