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IX. — The Formation of the Germinal Layers in Teleoslei. By George Brook, 

 F.L.S., Lecturer on Comparative Embryology in the University of 

 Edinburgh. (Plates XIII.-XV.) 



(Read 1st February 1886.) 



Introduction^ 



A little more than a year ago, I was led to conclude that the primitive 

 hypoblast in pelagic Teleostean ova was derived mainly from the unsegmented 

 protoplasm forming the floor of the segmentation cavity. In this respect 

 my results were mainly in harmony with the researches of Lereboullet (21), 

 Kupffer (18 and 19), Klein (16), Van Bambeke (3), and others, but 

 opposed to the more recent investigations of Henneguy (10), Hoffmann 

 (14), Kingsley and Conn (15), Ryder (24), Agassiz and Whitman (1), and 

 Cunningham (8.) At that time I adopted Agassiz and Whitman's name of 

 periblast for the subgerminal layer containing free nuclei, though I differed 

 from those authors in my idea of the mode of origin of the nuclei. As, how- 

 ever, I regard the tissues derived from this layer as belonging chiefly to the 

 parablastic group of His and Waldeyer, I propose to return for the present 

 to the older name of parablast. I was not enabled to trace the origin of this 

 layer in my earlier investigations, and thus failed to grasp its significance. 

 More recently I have had an opportunity of studying the development of 

 several other types, particularly the herring and cod. In the herring the ger- 

 minal mound is not formed until after fertilisation. Partly owing to this cir- 

 cumstance, and partly also to the early period at which the assimilation of yolk 

 commences, the eggs of the herring are particularly well suited for a study of 

 the parablast question. 



In order to understand the relation of the parablast to the yolk and to the 

 embryo, it will be necessary to give a detailed account of the structure of the 

 ripe unfertilised ovum, and of the early stages of development. In doing so, I 

 have compared my results with the earlier investigations of Boeck, Kupffer, 

 and Hoffmann. 



The Ripe Unfertilised Ovum. 



The ripe unfertilised ova of the herring vary considerably in size. This varia- 

 tion appears to be of a twofold nature. There is a certain variation found in 

 the size of ova from a single female, or in those from a particular spawning dis- 

 trict. There is also a variation in the average size of ova from one spawning 

 district, as compared with those of another. Kupffer (19) found that the ova 

 of the Baltic herring have a diameter usually varying between "92 and 1 mm., 



VOL. XXXIII. PART I. 2E 



