Cuap. II. FOLDINGS OF THE EMBRYONIC DISC. 5355 
within its outlines, and there only, that, in the order of Testudinata, the char- 
acteristic features of the type of Vertebrates are developed. From the beginning, 
the mode of formation and growth of this disc distinguishes the Turtles from the 
five lower classes of this type which have neither amnios nor allantois, namely, 
the Myzontes, the Bony Fishes, the Ganoids, the Selachians, and the Amphibians,’ 
though they belong to the same branch of the animal kingdom to which the 
order of Testudinata also belongs. Among the classes just mentioned, nothing like 
an amnios or allantois occurs, and the embryonic disc bears also different relations 
to the other parts of the egg. Thus we see, that, at the outset, this order 
exhibits characteristic features, distinguishing it from the classes below it, and show- 
ing its relation to the other orders of its class, and to the classes above it. 
Though prevailing universally, it is a mistaken view that the outlines of the 
embryonic dise are the boundaries of the animal, and that the yolk beyond and 
below is a mere appendage to it. We have already endeavored to show the fal- 
lacy of this theory in a former section, where it has been shown that the whole 
ege is, even from its first appearance, just as truly the animal as that part 
of the egg which is circumscribed within the region of the embryonic disc, when 
this dise becomes distinct from the rest of the yolk. We shall, therefore, speak 
hereafter of the whole egg as being the animal, and of those portions of the 
egg which are called the embryonic disc, the embryo, the amnios, the allantois, 
the vascular area, the area pellucida, the yolk mass, etc, as being so many differ- 
ent organs, or groups of organs, of one great organism. What we have said in 
a former section warrants us in the belief that we have taken the proper view 
of this subject; and, under these impressions, we will now proceed to investigate 
the formation, growth, and changes of the various organs which characterize this 
type of the vertebrate series. For obvious reasons we will begin with the embry- 
onie dise, as definite organization first makes its appearance there. 
The Embryone Disc. The area over which the embryonic disc extends is not so 
much marked by peculiarities of its own, as by the circular furrow (Pl. 10 fig. 14, @) 
of the germinal layer, which lies immediately beyond it; although it is true that 
between the two there is a slight difference in thickness, and in the intimate nature 
of their cellular constituents. However, the prominent feature that separates these 
two regions from each other is this intermediate furrow, which, as will soon be 
seen, is of great significance in relation to the development of an important 
organ, the amnios. The arching of the embryonic disc, like a blister on the 
surface of the yolk, renders the yellow color of the latter less visible, and the 
whiteness of the former more prominent. .In some instances, the formation of 
1 See p. 187. 2 See Sect. 6, p. 528. 
