Eelation of Yolk to Blastoderm in Telcostean Fish Ova. 187 



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XV. On the Relation of Yolk to Blastoderm in Teleostean Fish 

 Ova. By Geokge Brook, Esq., F.L.S., etc., Lecturer on 

 Comparative Embryology in the University of Edin- 

 burgh. 



(Read 20tli January 1886.) 



In January 1885 I contributed a paper to the Quarterly 

 Journal of Microscopical Science, in which the view was advo- 

 cated that the hypoblast in pelagic fish ova was not derived 

 from an invagination of the archiblast, as has usually been 

 supposed, but was formed almost entirely from the parablast. 

 This view has been received with a good deal of scepticism, 

 and indeed it is entirely at variance with the generally ac- 

 cepted theory of Teleostean development. IN^evertheless I was 

 from the first thoroughly convinced of the accuracy of my 

 statements so far as the material at my disposal was con- 

 cerned. Having during the past twelve months had increased 

 opportunities of studying the early stages of development of 

 several fishes, I have taken up the subject de novo. The 

 results of my inquiry lead me to reaffirm with greater 

 emphasis my former position. Hoffmann, Agassiz, Whitman, 

 and others, have during the past few years made a study of 

 the development of pelagic Teleostean ova. With similar 

 material to work upon, and in spite of the greater accuracy 

 of embryological methods, it is a remarkable fact that no two 

 authors are agreed on a single important point. It is very 

 improbable that there is such a great variation in the early 

 stages of Teleostean development as the various authors have 

 described. I do not propose for the present to enter into 

 any detailed discussion of the origin of the parablast. The 

 object of the present paper is to take up the question from 

 an entirely different point of view. I hope to show that a 

 careful study of the nature and behaviour of the yolk in 

 meroblastic ova, and a comparison with what obtains in 

 holoblastic types, will of itself throw very important light 

 on the much- vexed parablast question. Eor this purpose it 

 will be necessary to glance at the structure and growth of 

 the ovarian ovum. 



