THE EARLY DEVELOPMENT OF AMIA. 435 
tral lip the celenteron is no longer prominent. The closure 
of the blastopore is effected very much as in Lepidosteus 
or Acipenser, the constricting of its margin is the apparent 
cause of a greatly enlarged Randwulst; this ingrowth is at- 
tended by the protrusion of a slender yolk-plug (yp.), which 
on the blastopore’s closure appears to be largely withdrawn. 
A comparison of the gastrulation of Amia with similar 
stages of kindred forms may now be made. And this will be 
seen to become of especial interest since the intermediate 
characters of its gastrulation enable a far clearer understand- 
ing of this complicated growth stage of fishes than has yet 
been given. On the one hand the gastrula of Amia present 
decidedly Ganoidean features, while on the other its structures 
are clearly to be compared with those of the Teleost. An 
accompanying series of figures (figs. 6—13) enables compari- 
sons to be more readily drawn ; they present sections (nearly 
sagittal) of the earliest and of the latest stages of the gastrula 
of Lepidosteus, Acipenser, Amia, and Teleost; in the earliest 
stages the dorsal lip is coming to be formed; in the latest, 
the blastopore is closing. 
From the foregoing comparison it seems to the writer evident 
that well-marked transition exists in the structures of cor- 
responding stages. These he believes may, as in his figures, 
most conveniently be followed, starting with the archaic plan 
of gastrulation of Lepidosteus, passing to that (somewhat 
divergent) of the ancient sturgeon, thence to the more modern 
type of Amia, and finally to that of the highly specialised and 
recent Teleost. And it seems clear to him, furthermore, that 
the puzzling features of the gastrula of the specialised bony 
fishes (Teleocephah) may be interpreted as altogether due to 
a process of advancing or accelerated (precocious) development. 
Thus the advancing growth changes appear to be indicated in 
the following processes. 
1. The invagination tends to begin at an earlier stage when 
the blastodisc covers a smaller area of the egg’s surface, and the 
elements of the early gastrula tend to become relatively larger, 
and the roof of the segmentation cavity relatively thicker. 
