of the Fishery Board for Scotland. 



41 



cavity. The primitive hypoblast, which is almost entirely derived from 

 the parablast {i.e., from the vegetative pole), gives rise to the mesoderm, 

 and the secondary hypoblast (entoderm) remains as a single row of cells 

 in connection with the parablast. 



If I am correct in these conclusions, the similarity between the 

 development of teleosteans and amphibians (Eana) cannot fail to be 

 noted. The derivatives of the animal and vegetative poles are in both 

 cases practically identical. The secondary segmentation (buddijig) in 

 the parablast of teleosteans must then be regarded as the necessary 

 consequence of the relative distribution of protoplasm and yolk in the 

 vegetative pole. 



The primitive hypoblast, as here described for the herring, is precisely 

 homologous with that of Amphioxus. In both cases the primitive layer 

 gives rise to mesoderm, notocord, and true entoderm. 



The position here brought forward is one which I advocated over a 

 year ago, but from the nature of the material then at my disposal I failed 

 to observe the details of the process. Quite recently, Dr Ruckert (6), who 

 has been studying the behaviour of the parablast in Elasmobranchs, has 

 come to conclusions practically identical with those here advocated for 

 Teleosteans. The chief points of his paper, so far as our present purpose 

 is concerned, are as follows : — 



1. In Elasmobranchs the parablast arises with the first equatorial 

 division of the egg. 



2. In Torpedo the cells forming the roof of the segmentation cavity 

 (archiblast) give rise to the ectoderm. The nucleated cells of the 

 parablast (merocytes) give rise to the other embryonal layers. 



It remains to be seen how far our conclusions are confirmed by future 

 investigations, but it is an interesting fact that two authors working at 

 different groups of fishes should have quite independently arrived at 

 precisely the same conclusion. More particularly is this the case when 

 it is remembered that this conclusion is entirely at variance with the 

 generally-accepted theory on the subject. 



LIST OF LITERATURE REFERRED TO IN THIS PAPER. 



(1) KuPFFER, ' Z. Entw. d. Herings im Ei,' (Jahresb. d. Comm. z. wiss. 

 Unters. d. deutschen Meere, 1874-76, Berlin, 1878). 



(2) BoECK, ' Om Silden og Sildefiskerierne, &c. (Christiania, 1871). 



(3) Hoffmann, Z. Ontogenie d. Knochenfische (Amsterdam, 1881). 



(4) Klein, 'Early devel. of the Trout (Quart. Jour. Micros. Science, 1876). 



(5) Brook, ' On the Origin of the Hypoblast in Pelagic Fish Ova (Quart. 

 Jour. Micros. Science, 1885). 



(6) Ruckert, Keimblattbildung b. d. Selachier (Munich, 1885). 



EXPLANATION OF PLATES I and II. 



Figure 19 is taken from a drawing made for me by Mr J. T. Thompson, 

 M.B. ; for figures 20 to 24 I am indebted to Mr W. L. Calderwood ; while all 

 the others have been executed from living specimens by my assistant, Mr 

 Binnie. 



