440 SUMMARY OF CURRENT RESEARCHES RELATING TO 



of mesoderm segments 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. 



The nervous material which corresponds with the whole of the 

 cerebral heinisphei"e 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 thalamencephalon, and the conditions in 

 Actinopterygian Ganoids and Teleosts are similar. 



These are some of the general results of an exceedingly important 

 investigation. 



Development of Gymnarchus niloticus.* — Richard Assheton 

 describes the development of this Teleost, which belongs to the 

 Mormyridas, a primitive Malacopterygian family. His material was 

 collected in the Gambia by the late J. S. Budgett, and the memoir is the 

 first account of the development of any Mormyrid. The development is 

 on the whole typically Teleostean, but there are many interesting features. 

 The egg is large (10 mm. in diameter) and the development is very 

 rapid, the larva emerging upon the seventh day, whereas that of a trout 

 takes 35-100 days, according to the temperature. The elongated 

 embryo suggests that of an Amniote with almost typical " primitive 

 streak " ; the " archenteron " in so far as it occurs is more like that in 

 an early stage of Hypogeophis (Brauer) than a " Kupffer vesicle." In 

 the region of the primitive streak the hypoblast is continuous with the 

 yolk and the primitive streak as it is in Amphibia, and not separated as 

 it is in birds and mammals. There is a large mass of yolk, and the 

 larvae have very long gill-filaments hanging down in two blood-red 

 branches. 



The alimentary canal arises as a cleft among the hypoblast cells. 

 At an early stage — or perhaps from the outset— the pharyngeal region is 

 without a lumen. It does not acquire one until the larva is hatched. 

 There is one pair of true gill-clefts between the 6th and 7th 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-filaments on the 1st, 2nd, 3rd, and 4th 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 diverticulum of the 

 oesophagus a little to the left of the mid-dorsal line, has right and left 

 lobes ; its structure and 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 yolk 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 alimentary 

 canal just posterior to the oesophagus ; the pancreas is developed as 

 diverticula from the bile ducts (the constricted region just mentioned), 

 and these grow backwards to mingle with the " islands of Langerhans " 



* The Work of John Samuel Budgett (Cambridge, 1907) pp. 293-421 (6 pis., 

 80 figs.). 



