786 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The hypoblast appears to originate as in Trachinus ; the cells of 

 the periblast are pushed under the germinal disk until they cover 

 the whole floor of the segmentation-cavity ; cells absorbed from this 

 layer and free cells from the yolk contribute largely to form, if they 

 do not entirely form, the invaginated layer. Kupfifer's vesicle does 

 not appear until after the closure of the blastopore, and consists of a 

 solid mass of rounded cells. Early on the fourth day the oil-globule 

 was found to have an investing membrane binding it to the yolk; 

 this probably consists of hypoblast. As is usual with pelagic fish 

 eggs the circulation does not commence until some days after hatch- 

 ing ; like other fish whose ova are pelagic Motella spends a long time 

 (six days) vs^ithin the ovum ; the author emphasizes this point by 

 giving a list of the principal species of fish with pelagic ova and the 

 date at which they have been observed to leave the egg. 



Development of the Salmon.* — Mr. J. A. Eyder describes certain 

 features of the development of the land-locked salmon (Salmo salar 

 var. sebago) viz. the arrangement of the blood-vascular system at the 

 time of hatching ; some of the impairments which this system suffers 

 when the young fishes are under the care of the fish-culturist ; and 

 the development of the fins. 



Spawning of the Cod.t — The observations made by Prof. Cossar 

 Ewart and Mr. G. Brook justify the conclusion of Sars that the spawn 

 is shed while the fishes are swimming about freely in the water, and 

 that the eggs are fertilized at, or as they rise to the surface, this 

 being facilitated by the position of the micropyle, which is always 

 found in the lower hemisphere of pelagic fish ova. The eggs and 

 milt are of less specific gravity than the sea-water, and consequently 

 float. 



For some time before the first eggs reach maturity, and during the 

 early part of the spawning period, the fish not only refuse food, but 

 give up their regular movements around the tank and swim about in 

 small groups or rest together at the bottom, swimming and resting 

 alternately. The activity of the males was specially evident at dusk 

 and in the early morning, and it was apparently during these periods 

 of activity that the eggs were shed and fertilized. The males swim 

 indiscriminately among the females, sometimes over, sometimes under 

 them, fertilizing the water through which the shed eggs are slowly 

 rising to the surface. Eggs were pressed from a ripe female and 

 fertilized artificially. The females, like the Salmonid^, are capable 

 of withholding the flow of ripe eggs to a certain extent. A limited 

 number only are ripe at one time, and if the unripe be forced out 

 they sink to the bottom and are incapable of being fertilized. 

 Whether fertilized or not the ripe eggs float immediately after ex- 

 trusion ; but in the latter case they die and sink to the bottom in 

 twelve to fourteen hours. 



In perfectly still water the eggs float in a dense mass ; -when 



* Proc. U.S. Nat. Museum, viii. (1885) pp. 156-62 (1 pi.). 



t Ann. Eept. Fishery Board for Scotland, 1884. See ' Nature,' xxxii. (1885) 



p. 282. 



