of Edinburgh, Session 1884-85. 
169 
than elsewhere. The inflected or invaginated layer when it appears 
constitutes part of the blastoderm, and is situated between the rest 
of the blastoderm and the yolk mass. The superficial layer of the 
yolk mass beneath the blastoderm is protoplasmic and nucleated, 
though not divided into cells. This layer is called the periblast, 
and at a later stage forms a continuous covering all over the yolk 
mass, from which it is never distinctly separated. The thickened 
radius of the blastoderm is the first indication of the embryo, and 
is known as the embryonic rudiment. As the blastoderm grows 
over the yolk, the germinal ring, exclusive of the embryonic rudi- 
ment, remains of the same breadth, but the rudiment grows in 
length at its posterior end, while the anterior end, that which is at 
first situated near the centre of the blastoderm, does not alter its 
position. The posterior end of the embryonic rudiment is always 
at the edge of the growing blastoderm. Towards the close of the 
process of envelopment of the yolk by the blastoderm, the germinal 
ring is gradually absorbed into the embryonic rudiment, and finally 
the whole of it is seen to form part of the dorsal region of the 
embryo. In this way the whole of the invaginated layer comes 
to lie beneath the axis of the embryo. There can be little doubt 
from what is known concerning the development of other vertebrate 
forms, that the invaginated layer ultimately forms the dorsal wall 
of the intestine, the floor of which is derived from the periblast. 
It was maintained some years ago by the German embryologists, 
Ilis and Rauber, that the explanation of the formation of the dorsal 
region of the embryo out of the germinal ring was, that the two 
halves of the ring were gradually fused together by concrescence 
taking place at the point where they were continuous with the 
posterior end of the embryonic rudiment, and that the edge of the 
blastoderm was the blastopore, which was thus situated along the 
median dorsal line of the embryo. This view is generally accepted 
by more recent students of Teleostean development, but no examina- 
tion has been made of the theory or process in detail. His and 
Rauber, in their exposition of the concrescence theory, connected it 
with the discussion of facts concerning the development of the 
medullary folds in Elasmobranchs. It seems to me important that 
it should be clearly understood that the process of concrescence 
which undoubtedly takes place in Teleostean ova is simply the closing 
