10 Bashford Dean Memorial Volume 



essentially like the median fins and brought weighty evidence in support of the Thacher- 

 Balfour fin-fold theory of the origin of the paired limbs, in opposition to Gegenbaur's 

 view that the paired fins had arisen from feather4ike "archipterygia," like those of the 

 existing lung-fishes. 



Subsequent papers and memoirs of his on Cladoselache and on sharks as ancestral 

 fishes appeared at intervals up till 1902. He also inspired his student, Raymond C. 

 Osburn, to prepare a detailed critique of the opposing ''fin-fold" and "archipterygial" 

 theories and supplied him with the material which demonstrated that during the course of 

 ontogeny the skeleton of the paired fins has a similar origin to that of the median fins and 

 a radically different origin from that of the branchial arches. 



The Cladoselache material described by Dean (1902, 1909) was also remarkable for 

 containing well fossilizied muscle segments and even red muscle fibres, as well as the 

 glomeruli of the kidneys. Under the general heading of "Studies on Devonian Sharks'" 

 we may also note the fact that after Professor Newberry's death Dean edited one of the 

 latter's memoirs on American Palaeozoic fishes, containing descriptions of various forms of 

 dental plates and fin spines of elasmobranchs (1897)- Dean's 1907 paper on Acanthodian 

 sharks was in the nature of a general morphological review of conspicuous members of 

 that group and contained many valuable sketches made in European museums. 



STUDIES ON THE EMBRYOLOGY OF SHARKS AND GANOIDS 



1893-1909 



Dean's interest in the Palaeo2;oic sharks led him into his world-wide search for the 

 embryonic stages of the Port Jackson Shark (Cestracion or Heterodontus), the Japanese 

 Frilled Shark (Chlamydoselachus), and other archaic survivors of the class, in order to 

 compare the findings of embryology with those of palaeontology. This division of his 

 researches resulted also in the accumulation through the years of a great series of beauti- 

 fully executed plates on the development of these forms, but most of these plates remained 

 unpublished at the time of his death. Nevertheless he published various brief papers 

 dealing with the eggs and cleavage stages of the sharks as follows: "Reminiscence of 

 Holoblastic Cleavage in the Egg of the Shark, Heterodontus (Cestracion) japonicus 

 Macleay." (1901), "The Early Development of Sharks from a Comparative Standpoint" 

 (1901), "A Preliminary Account of Studies on the Japanese Frilled shark, Chlamydo' 

 selachus" (1903). 



From the time of his early association with the fish-culturists, he lost no chance of 

 securing embryological material among the fresh-water Ganoids. Beginning in 1893, 

 several papers deal with the spawning habits and early developmental stages of the sturgeon 

 (Acipenser, 1895), garpike (Lepidosteus, 1895) and bowfin [Amia, 1895, 1896). These 

 studies in turn led to some comparative observations on the gastrulation of teleosts (1895) 

 and finally (1909) to a comparative study of "The Plan of Development in Series of Forms 

 of Known Descent and its Bearing upon the Doctrine of Preformation." His general 

 conclusion in this brief but important paper was that apparent preformation in the 

 embryo arises from the acceleration of the process of epigenesis. 



