EMBRYONIC DEVELOPMENT OF THE TELEOSTEANS. 41 



A CoNTKiBUTioN to the HiSTOEY of the Embryonic Develop- 

 ment of the Teleosteans. By Edouard Van Beneden, 

 Professor in the University of Liege. (With Plate IV,) 



During my stay at Villafranca, in August and September, 

 1874, 1 had occasion to make some observations on the deve- 

 lopment of an osseous fish. I applied myself chiefly to the 

 study of the segmentation and endeavoured to elucidate the 

 much discussed question of the origin and mode of forma- 

 tion of the embryonic layers. I found daily, in the midst of 

 the produce of my fishing with Miiller^s net small pearl-like 

 bodies, colourless and perfectly transparent, floating freely near 

 the surface of the water; the diameter of these little hyaline 

 spheres scarcely exceeded that of a large Noctiluca, and it was 

 only after I had examined them with a lens that I discovered 

 that I had before me the embryos of an osseous fish. 



Several times the fishermen brought me masses of a gelati- 

 nous appearance, taken on the surface of the sea and formed by 

 hundreds or thousands of agglutinated eggs. These eggs pre- 

 sented all the characters of those of which mention has just 

 been made ; they had the same transparency, and the same com- 

 position. All the eggs in the same mass were always found to 

 be in the same stage of development. This circumstance 

 greatly facilitates the study of the successive phases of develop- 

 ment. The eggs die very rapidly on the microscope-stage and 

 consequently one cannot witness the progress of the development 

 under one's eyes in one and the same egg. But since all the 

 eggs of a given mass develope simultaneously, it is always pos- 

 sible to determine the time which has been necessary for the 

 production of modifications which have arisen since the moment 

 when the last eggs examined were detached from the agglo- 

 meration. 



In all the masses of eggs which were brought to me the 

 eggs were always in course of cleavage or else they contained 

 very young embryos. I have never found embryos on the point 

 of hatching nor even embryos provided with primordial vertebrse. 

 On the other hand, I have never found in the free isolated con- 

 dition eggs in course of segmentation nor even embryos young 

 enough to be utilised in the study of the formation of the germ- 

 layers. 



It seems therefore probable that the agglutinated eggs come 

 from the same fish as those which were taken isolated. It 

 appears that the eggs laid in mass remain for some time adherent 

 one to another and afterwards separate and then float free from 

 all adhesion on the surface of the sea. 



