85 



THE 

 ORGANISATIOX OF THE TELEOSTEAN EGG. 



BY ALEXANDER MEEK. 



This is a preliminary paper. It is meant to serve as an intro- 

 duction to a fuller account with especial reference to the embryology 

 of the cod, which I hope to be able to publish by-and-by. 



Notwithstanding the vast amount of work which has been 

 done on the development of the Teleostei, the meanmg of the 

 processes leading to the establishment of the germ layers continues 

 to be obscure. A tentative nomenclature has been found to be 

 necessary, but no one is satisfied with any of the theories which 

 have been advanced to explain the " blastodisc " and the " sub- 

 germinal '' and " peripheral periblast." A fresh exploration of the 

 facts is therefore called for. 



The Fertilised Ovum. — Apart from the capsule, the origin 

 and nature of which I am not at present to discuss, the relatively 

 small meroblastic ovum of pelagic Teleostei consists of a nucleus 

 near the animal pole, surrounded by a conspicuous polar mass of 

 cytoplasm, sharply marked off from the rest of the cytoplasm which 

 is distended by the yolk. The polar cytoplasm is contmued around 

 the yolk as a fine pelHcle, the peripheral protoplasm, and both the 

 polar and peripheral protoplasm are continued by fine strands of 

 protoplasm into the yolk, where a network of protoplasm is formed. 

 The network may be seen at all stages of segmentation, and also 

 minute swellings or knots which increase in size peripherally. The 

 latter point to a continuation of the process of streamhig which has 

 led to the conspicuous differentiation of the yolk and the protoplasm. 

 The process of streaming was carefully described by Ransom * and 

 by Kowalswsky f for fresh water fish, and by Ryder { and by 

 Agassiz and Whitman || for pelagic sea fish. The result of the 

 streaming is not merely to produce the obvious polar mass and the 

 peripheral layer of protoplasm, but, as will be seen, to orientate 

 both definitely with respect to the structures which are to arise from 

 them. 



* 1867, Phil, Trans. Roy. Soc, v., 157. 



t 1886, Zeit. f. wiss. Zool.,'Bd. 43. 



:;: 1885, Rep. Fish. Comm., U.S. 



I, 1889, Mem. of the Mus. of Comp. Zool., Harvard, iv., 14. 



