150 SUMMARY OF CUERENT RESEARCHES RELATING TO 



follicles arise from a ridge-like thickening of the mucous membrane, but 

 in the adult those on the palate predominate over those of the choanse. 

 In the toothless toad there is a distinct dental strand on the upper jaw. 

 In the frog there is in the development of the dental strand and of the 

 teeth, an adumbration of the state of affairs in Mammals. But we 

 cannot do more than indicate the general nature of the author's 

 investigation. 



Suggestions from the Embryology of Fishes. *^ — J. Graham Kerr 

 discusses the embryology of certain of the lower fishes, and its bearing 

 upon Vertebrate morphology. Perhaps Vertebrate embryology has been 

 too much influenced by deductions from forms (e.g. Selachian and chick) 

 with a large mass of food-yolk. 



The process of gastrulation in FoJypterus is interesting as a trans- 

 itional condition between that found in Ainphw,nis and that characteristic 

 of Amphibians. In Lepidosiren there is a decided advance towards the 

 type of gastrulation found in the Elasmobranchs. 



Various considerations make it proba])le that Vertebrates, Annelids, 

 Arthropods, and Molluscs are offshoots of a common triploblastic stem. 

 On the other hand, \arious points in the development of Vertel)rates, 

 e.g. the apparent traces of a once present slit-like protostoma, forming 

 a continuation of the blastopore forwards along the mid-line of the 

 medullary plate, suggests an actinozuon-like ancestor. 



External gills, conspicuous features in the development of Amphi- 

 bians, of the two existing Crossopterygians, and of two of the three 

 surviving Dipnoans, are probably homologous structures of great 

 antiquity. Their very early appearance, the importiince of their blood- 

 supply — the main aortic arch passing into them — and the remarkable 

 constancy of their position in relation to the upper end of the visceral 

 arch, are among the facts which lend importance to these structures. 



It is suggested that, if the gill-clefts were, in their first functional 

 stages, clefts (and not pouches), their primary significance may have 

 been in connection with the pumping of water over the external gills. 

 Secondarily, they may have become directly respiratory by a spreading 

 of respiratory ectodermic epithelium along the walls of the clefts. 



The homology of the swim-bladder of Crossopterygians and Dipnoans 

 with lungs seems now undeniable. The hydrostatic function of the 

 swim-bladder and the dorsal migration of its connection with the 

 pharynx is probably secondary. 



The only paired organs of lower Vertebrates which fulfil the two 

 qualifications of pre-existing structures from which paired limbs might 

 have evolved, viz. projection and muscularity, are the external gills. 



In Polypterus, Lepidosiren, and Protopttervs, the brain shows at an 

 early stage a division into two regions, the posterior giving rise to the 

 hind brain region of the adult, the other to the whole of the Imxin 

 anterior to this. The division into " rhombencephalon " and cerelnaim 

 is probably primitive. From a study of the same three forms evidence 

 is found supporting the view that the cerebral hemisphere region is 

 primitively pau-ed, and absolutely against the view that the hemispheres 



* Proc. Roy. Phys. Soc. Edinburgh, xvi. (190fi) pp. 191-215. 



