July 26, 1912] 



SCIENCE 



121 



however, will on the whole prove the more ac- 

 ceptable to morphologists, since they do not 

 show the utter disregard of certain funda- 

 mental principles of embryology so character- 

 istic of Professor Gaskell's theory. 



Professor Patten, as is well known, traces 

 the vertebrate phylum back to an arachnid 

 ancestry, represented by such forms as the 

 Eurypterids, and thence traces it upwards 

 through the Ostracoderms to the primitive 

 arthrodire, dipnoan and amphibian forms. 

 It will not he possible here to discuss the va- 

 rious homologies of arachnidan and vertebrate 

 structures proposed in the book, such as the 

 identification of the arachnidan limbs with 

 the vertebrate external gills, of the arachnidan 

 liver lobes with the vertebrate pharyngeal 

 pouches and thyreoid, of the lemmatochord 

 with the notochord. ISTor can more than men- 

 tion be made of the comparison of the lateral 

 and parietal eyes of the arachnids with the 

 corresponding structures in vertebrates, or of 

 the suggestive similarity of the endocranium 

 and branchial cartilages of Limulus to those 

 of the vertebrates. But attention may be 

 especially directed to two points that seem to 

 be crucial for the theory, namely, the homolo- 

 gies of the surfaces of the body and the mode 

 of evolution of the vertebrate brain. 



The hn?mal and neural surfaces of the verte- 

 brate descendant are homologized with the 

 same surfaces of the invertebrate ancestor 

 whose appendages have migrated hsemally 

 until their basal portions form the branchial 

 arches and whose mouth has been supplanted 

 by a new one, so that the stomodseum no longer 

 perforates the central nervous system. A por- 

 tion of the original stomodseum has become the 

 ependymal lining of the third ventricle, 

 another portion of it is represented by the 

 infundibulum and hypophysis, and the new 

 mouth is formed from what Professor Patten 

 terms the cephalic navel, the region where in 

 Limulus the blastoderm completes its invest- 

 ment of the yolk and which is identified with 

 the dorsal organ of other arthropods. The 

 cause of the closure of the original mouth 

 Professor Patten finds in the constriction of 

 the stomodiBum by the consolidation and en- 



largement of the cephalic neuromeres and the 

 evident difficulty in the way of imagining 

 such a " cataclysmic metamorphosis," as Pro- 

 fessor Patten rightly calls the closure of one 

 mouth and the opening of another, he en- 

 deavors to avoid by the suggestion that it took 

 place " during the embryonic, or larval period, 

 the increasing volume of the yolk sphere ma- 

 king . . . (it) possible." In other words, he 

 suggests that the closing of one mouth, the 

 development of another, the transfer of the 

 oral arches to the hasmal side and the appear- 

 ance of true gill clefts all took place in the 

 course of the life history of a single genera- 

 tion! Or should it be of a single individual? 

 With regard to the second point referred to 

 above, Professor Patten holds that the entire 

 arachnidan nervous system is equivalent to 

 the vertebrate brain, the spinal cord being a 

 later development which does not and prob- 

 ably never did show a division into neu- 

 romeres. But, it may be remarked in passing, 

 in the vertebrate brain thus formed are in- 

 cluded the hypoglossal segments, the hypo- 

 glossal nerve being the result of the segrega- 

 tion of special fibers from the vagus complex 

 supplying the gills. The hypoglossal seg- 

 ments are, accordingly, primary constituents 

 of the vertebrate brain, a view entirely at 

 variance with the results of vertebrate mor- 

 phology. The arachnidan fore brain consists 

 of that portion of the brain situated in front 

 of the stomodffium and is composed of three 

 neuromeres. The first of these is associated 

 with the olfactory organ and represents the 

 olfactory lobes ; the second is associated with 

 the lateral eyes and its mushroom bodies be- 

 come the cerebral hemispheres, while behind 

 the third neuromere lies the stomodseal com- 

 missure, which is the representative of the 

 vertebrate cerebellum! The medulla oblon- 

 gata corresponds, however, to the abdominal 

 or branchial portion of the nervous system of 

 Limulus and, therefore, there is no correspond- 

 ence whatever between the neural and haemal 

 surfaces of the vertebrate brain as far back as 

 the region of the cerebellum, the neural sur- 

 face of all this portion of the brain, including 

 the hemispheres, the roof of the tweenbrain, 



