314 



NA TURE 



[August 6, 1908 



development in Polypterus show frequent striking 

 resemblances with what occur in Dipnoans and in the 

 lower Amphibia. I believe tliese resemblances are 

 sufficient by themselves to indicate the probability that 

 the Teleostomes, the Dipnoans and the Amphibians 

 have arisen in phylogeny from a common stem, which 

 would in turn probably have diverged from the ances- 

 tral Selachian stock. The ancestors of the Amniota 

 probably diverged either about one or about several 

 points from the region of the stem common to Dipnoi 

 and Amphibia." 



(2) The external gills develop in Polypterus exactly 

 as they do in Lepidosiren and Protopterus and in the 

 more primitive Amphibia (Urodela and Gymnophiona), 

 i.e. each one arises as an outgrowth from the outer 

 side of the visceral arch (in this case hyoidean), com- 

 posed of mesenchymatous core with ectodermal 

 covering. They appear before the perforation of the 

 gill-clefts, and are probably organs of great antiquity. 

 Tiie respiratory epithelium of the gill-clefts has arisen 

 by a spreading inwards from the ectodermal respira- 

 tory epithelium of the external gills. 



(3) It may be that paired limbs are homodynamous 

 with external gills in which the potential motor 

 function has been accentuated. 



(4) As Budgett showed, the condition of the fin- 

 skeleton in the 30 mm. larva of Polypterus indicates 

 its close relationship to the type of uniserial fin- 

 skeleton occurring in sharks. Prof. Graham Kerr 

 briefly re-states the hypothesis that both can be 

 referred back to a primitive biserial archipterygium 

 like that of Ceratodus. 



(5) In the evolution of the head there has been a 

 varying amount of displacement in an anteroposterior 

 direction of the relative positions of mesoderm seg- 

 ments and visceral pouches, and it is suggested that 

 the enterocoelic pouches were once wholly posterior 

 to the visceral pouches, and that the two structures 

 are really homodynamous. 



(6) The nervous material which corresponds with 

 the whole of the cerebral hemisphere in the higher 

 forms — including the pallium or mantle — lies in 

 Polypterus in the thickened wall of the thalamen- 

 cephalon. What is ordinarily called the pallium in a 

 crossopterygian is simply the roof of the thalamen- 

 cephalon, and the conditions in actinopterygian 

 ganoids and teleosts are similar. 



In the river Gambia Mr. Budgett found the " nest " 

 of Gymnarchus niloticus, a primitive and at the same 

 time specialised teleostean, belonging to the family 

 Mormyridae. He secured a fine series of the eggs at 

 different stages, and these have been described by 

 Mr. Richard Assheton in a remarkably fine memoir, 

 which is the first contribution to the embryology of 

 Mormyrids. Mr. Budgett gave an account of the 

 floating nest with its thousand large eggs like amber- 

 beads. The development is extraordinarily rapid, for 

 the eggs hatched in seven days, and in eighteen days 

 the young fry left the nest three inches long. Mr. 

 Budgett thought that the development was " exceed- 

 ingly shark-like," but this has not been borne out by 

 Mr. Assheton 's work. There is a large mass of yolk, 

 and the larvae have very long gill-filaments hanging 

 NO. 2023, VOL. 78] 



down in two blood-red branches, but the development 

 is on the whole typically teleostean. We can only 

 refer to a few of the many interesting features. 



The alimentary canal arises as a cleft among the 

 hypoblast cells. At an early stage — or perhaps from 

 the beginning — the whole of the pharyngeal region is 

 without a lumen, and it does not get one until after 

 hatching. There is one pair of true gill-clefts 

 between the sixth and seventh visceral arches ; the 

 other " gill-clefts " of embryonic life are invaginations 

 of the ectoderm which undermine the visceral arches. 

 There are long external uniramous gill-tilaments upon 

 the first, second, third, and fourth branchial arches, 

 which shrivel after the operculum has grown over 

 them, excepting the proximal ends which give rise to 

 the permanent gills. The whole apparatus is lined by 

 epiblast from first to last. 



The air-bladder, which arises as a single diverti-; 

 culum of the oesophagus a little to the left of the' 

 mid-dorsal line, has right and left lobes, and is 

 extremely lung-like. Its structure, its vascular 

 supply, and the habits of the fish all point to its use 

 as a lung. The yolk-sac is to be regarded as an 

 appendage of the liver — due to the accumulation of 

 volk in that part of the egg' which normally becomes 

 the liver. The gall-bladder and liver arise by the 

 constriction off of a large ventral recess of the alimen- 

 tary canal (just posterior to the oesophagus) ; the 

 pancreas is developed as diverticula of the bile-ducts 

 (the constricted region just mentioned), and these 

 grow backwards to mingle with the " islands of 

 Langerhans " tissue and even with the spleen; the 

 islands of Langerhans arise very early as a solid mass 

 of epithelial tissue which becomes broken up by the 

 splitting of the mesenteric artery. 



There are certain features which suggest an earlier 

 condition of teleostean evolution than is the case with 

 other members of the class the development of which 

 has been studied hitherto, and Mr. Assheton inquires 

 whether Teleosteans may not be descended from a 

 proto-amphibian race. He refers to the amphibian- 

 like character of the lips of the blastopore, to the 

 vestige of neural tube formation, to various features 

 in the development of the excretory system, to the 

 lung-like and vestigially double air-bladder, to the 

 trace of an auricular septum and the suggestion of a 

 double circulation, to the large size of the aortic arch 

 of the fourth visceral arch, and to the peculiar char- 

 acter of the gill-clefts, filaments, and arches. 



Is it possible that the teleosteans descended from a 

 proto-amphibian stock more amphibian than the 

 Dipnoi, that they owe their position of prestige to 

 having served an apprenticeship in a less regular and 

 constant environment, that a mutation led to a return 

 to strictly aquatic habits? With questions such as 

 these, Mr. Assheton relieves his statement of the facts 

 of the development of Gymnarchus. 



Mr. Assheton also reports on sundry teleostean 

 eggs and larvas which Mr. Budgett collected in the 

 Gambia, and Dr. E. J. Bles contributes descriptions of 

 some stages in the development of three Anura — 

 Paludicola fuscomaculata, Hemisiis marmoratiim, and 

 Phylloniedusa hypochondrialis. The last paper re- 

 minds us of what will strike everyone who looks over 

 this stately volume ; we refer to the fine illustrations. 

 Mr. Budgett was a skilful draughtsman, Mr. Asshe- 



