44 W. B. SCOTT AND HENRY F. OSBORN. 



of the Batrachia. It is, as might be expected, intermediate 

 between the two types in many ways. In the Lamprey the 

 division into two layers does not occur until a comparatively 

 late period, some time after the larva has been hatched, while 

 in the Newt it occurs as early as Stage c. In the Frog it 

 is found from the first. Another respect in which the Newt 

 is intermediate is the histological character of the layers. 

 The Elasmobranch Fishes in this respect present an inter- 

 mediate condition between the Lamprey and the Newt. In 

 them also the epiblast is primarily single ; the first change 

 consists in the part which will give rise to the central 

 nervous system, becoming several cells thick, but presenting 

 no distinction into two layers. Eventually, later than in the 

 Newt, earlier than in the Lamprey, the epiblast divides into 

 mucous and epidermic layers. Both layers seem to enter into 

 the formation of the organs of sens*, while in the Amphibians 

 the sense organs are formed exclusively, or almost so from the 

 mucous layer, and in the Lamprey they are derived from the 

 epiblast before its division into the layers. 



These facts put us in a somewhat favorable position for 

 the solution of the question as to whether the single- or 

 double-layered epiblast is the primitive condition. We are 

 decidedly of the opinion that the conclusion drawn by Mr. Bal- 

 four on p. 100 of his book on the Elasmobranchs is the correct 

 one, viz. that the single-layered epiblast is the more primitive 

 condition. He was not aware at that time of the difference 

 existing between the Frog and the Newt in this regard, and 

 so attributed the double layer to the Amphibians generally. 

 But, as we have seen, it is confined to the Batrachians, a 

 much more restricted group, and is, perhaps, also found in 

 Osseous Fishes. Besides these it is found in no other groups 

 of the animal kingdom, and, as Mr. Balfour points out, it is 

 more probable that a particular feature of development should 

 be thrown back to an earlier period than for the distinction 

 between the two layers to be absolutely lost, and then to 

 reappear at a later stage. This apriori consideration receives 

 a great deal of support from the facts of the development of 

 the Newt. By its aid we are enabled to arrange a series of 

 steps of advancing differentiation of the epiblast from Amphi- 

 oxus through the Marsipobranchs, the Elasmobranchs, and 

 the Newt, to the Batrachians. 



The steps of this progression have been already stated, but 

 it may be well to summarise them. (1.) Amphioxus has an 

 epiblast consisting at first of short columnar cells in a single 

 TOW. These afterwards begin to flatten out, and in the adult 

 are very much flattened, hut never constitute more than a 



