FORMATION OF THE GERMINAL LAYERS IN TELEOSTEI. 225 



the origin of the nuclei in the parablast, but is inclined to regard the 

 hypoblast as resulting from an invagination of the outer, instead of the inner 

 portion of the epiblast. Regarding the latter point, Cunningham may be 

 said to stand almost alone in supporting Haeckel's view of the formation of 

 the Teleostean gastrula, Kingsley, as already stated, having ceased to hold 

 that view. The general question of the origin and significance of the pan- 

 blastic layer in meroblastic ova has not received much attention in this 

 country. Balfour (4 and 9), while accepting to a certain extent His's view 

 as to the development of free nuclei in the surface yolk of Elasmobranchs, and 

 in the white yolk underlying the blastoderm of the chick, does not accept his 

 terminology. According to Balfour, a number of cells are formed in the upper 

 strata of the yolk, which unite with the cells of the blastoderm during the 

 processes of invagination and differentiation of the germinal layers, but these 

 are apparently of only secondary importance. If the so-called " germinal 

 wall " of the chick embryo is a portion of the layer here termed parablast, 

 and there seems no room for doubt on this point, Balfour certainly held 

 that certain portions of all the germinal layers may chiefly or partly be 

 produced from this layer. Speaking of the differentiation of the layers in 

 the area vasculosa of the chick, he says : — " The mesoblast and hypoblast of 

 the area opaca do not arise by simple extension of the corresponding layers 

 of the area pellucida ; but the whole of the hypoblast of the area opaca, and a 

 large portion of the mesoblast, and possibly even some of the epiblast, take 

 their origin from the peculiar material which forms the germinal wall, and 

 which is continuous with the hypoblast at the edge of the area opaca." In 

 his latest contribution to the parablast question, His (13) withdraws from the 

 position which he formerly held in respect to the cellular character of the 

 white yolk spheres, and consequently of the derivation of nuclei and cells 

 from them. According to his view, the relation of the parablast to the 

 embryo may be shortly summarised in the following manner : — The segmented 

 blastoderm (archiblast) gives rise to the three primary germinal layers, 

 epiblast, mesoblast, and hypoblast, but these only give rise to archiblastic 

 tissues. 



The epiblast gives rise to the epidermis and the true glands derived from 

 it, and to a part of the epithelium of the digestive tract, as well as to the 

 nervous system. 



The hypoblast forms the rest of the epithelium of the digestive tract, and 

 the glands belonging thereto. 



The mesoblast (or, more properly speaking, that portion of it which is 

 derived from the archiblast) gives exclusively smooth and striped muscles, 

 together with the epithelium of the urogenital tract. Mesoblast also gives 

 r ise to the primitive clothing of the ccelom; but, according to His, this is 



