DEVELOPMENT OF THE TADPOLE. 205 



illustrated by sectional diagrams, is, that the mesoblast forms a 

 continuous sheet around the ovum, under the epiblast, and that 

 in the axial dorsal line a central chord becomes separated to form 

 the notochord. Calberta, however, states, that when the meso- 

 blast is fully formed, and separated from the hypoblast, it does 

 not form a continuous sheet, but that it is broken in the dorsal 

 axial line, and the two portions separated from each other by a 

 ridge of hypoblastic cells. It is this hypoblastic ridge, he says, 

 and not a layer of cells from the mesoblast, which becomes differ- 

 entiated into the notochord. 



Balfour says that his own observations are in favour of the 

 latter statement, as the mesoblast never appears to form an abso- 

 lutely continuous sheet. When such eminent embryologists differ, 

 it would be the highest presumption on my part, with my very 

 limited number of sections, to offer a decided opinion on so 

 extremely delicate a matter ; but I may state that when cutting 

 my earlier sections many of them were rejected because the layer 

 of mesoblast was never found perfect, and as my time for mount- 

 ing is very limited, I was obliged to retain only those which, from 

 the observations of previous observers, were considered most nearly 

 accurate. As, however, in other types there appears to be no 

 doubt that the notochord is really derived from the hypoblast, it is 

 very probable that the same thing takes place in this. As no 

 invertebrate animal possesses a notochord, it will be seen that the 

 study of this structure is of the highest importance in an embryo- 

 logical sense. 



It would appear, from the observations of Gotte, Schneider, 

 Balfour, and others, that the structure of the notochord in the 

 Teleostei, Elasmobranchii, etc., is distinctly different from that 

 of the Amphibians, the former possessing an elastic sheath, called 

 the membrana elastica externa, and the latter being devoid of this 

 structure. My own sections, at the date given, and as roughly 

 drawn in PI. XIX., Fig. 2, show that the notochord at this period 

 consists of three parts : — Firstly, a central cellular portion ; 

 secondly, a delicate layer of cells; thirdly, a strong, fibrous, 

 elastic sheath. These we will describe in detail. 



The central cellular portion consists, at its very anterior 

 extremity, of what appears to be a two-celled layer. From a con- 



