460 W, B. SCOTT AND HENRY F. OSBORN. 



single row. The medullary plate is the only epiblastic 

 development which consists of more than one row of cells. 

 This fact alone is of considerable weight in the question we 

 are considering; and it should be borne in mind throughout 

 the discussion that, in the most primitive vertebrate known, 

 the epiblast \% permanently single-layered. Into the peculiar 

 method of the formation of the cerebro-spinal axis we need 

 not enter. 



(2.) In the Lamprey the epiblast does not divide until very 

 late; \xi fact, not before the embryo has for some time been 

 hatched (see Calberla, loc. cit.^ p. 264). This change 

 takes place, however, in the region of the spinal cord before 

 that organ has been formed, just as is the case in Amphioxus. 

 The development of the nervous axis presents some pecu- 

 liarities of a secondary nature. The sense organs are formed 

 from the undivided epiblast. 



(o.) The epiblast in the Elasmobranch Fishes separates 

 into two layers much earlier than it does in the Lamprey, but 

 still comparatively late in embryonic life, some time after the 

 medullary canal has been completely closed, and several of 

 the visceral clefts have appeared. According to Mr. Balfour 

 it takes place at a stage slightly younger than K. The two 

 layers are at first composed of flattened cells, but those of 

 the inner stratum soon become columnar. ''Both layers 

 apparently enter into the formation of the organs of se7ise" 



(4.) In Triton the epiblast, though at first single, divides 

 into its two parts at a very early stage, some time before 

 the closing of the medullary canal (Stage c). When once 

 formed the mucous layer becomes the active one and enters 

 almost exclusively into the formation of the sense organs. 

 So far as we are aware this is the only case as yet known in 

 which there is a primitively single epiblast dividing early 

 and delegating all its activity to one layer. It shows an 

 approximation to the state of things found in the Frog. 



(5.) In the Batrachia this is carried one step further and 

 the two layers are distinguishable from the very first, even 

 the roof of the segmentation cavity being double. The 

 mucous or nervous layer, as in the Newt, enters alone into 

 the formation of the organs of sense, &c. In short, almost 

 the only difi'erence in the matter of epiblast between the two 

 classes of Amphibia lies in the time of its division. 



Now, we are very far from asserting that these forms 

 we have been considering represent the line of descent of the 

 Batrachia ; but we are decidedly of the opinion that they 

 exhibit the steps of the process by which the epiblast of 

 that group has reached its present complication. For 



