316 
E. W. MACBRIDE. 
ordinarily like those of Urodele and Amiran Ainpliibia. 
Since a very similar method of development is found also in 
such old-fashioned types of fish as the Dipnoi, Polypterus, 
and the sturgeon, we may with some probability conclude that 
this method was once universal amongst Yertebrates, and 
persisted till the first invasion of the land had been accom- 
plished. Apuultitude of papei’s has appeared on the develop- 
ment of the Amphibia, but the careful and exhaustive work 
of Bracket ^(6) on the comparison of the development of 
Siredon and of the frog* will, I think, for some time represent 
the last word on the subject. Bracket finds that at the com- 
pletion of segmentation endoderm and ectoderm are not 
delineated, that although the lower pole of the egg is occu- 
pied by large cells with large yolk-giaunles and devoid of 
pigment, whilst the upper pole is the seat of small cells with 
small yolk-granules and pigment, yet there is a gradual 
passage from tlie one type of cell to the other. As 
in Amphioxus the differentiation of ectoderm and endo- 
derm accompanies a process of growth and multiplica- 
tion of cells, some of the daughter-cells which result there- 
from being endoderm with large yolk-granules, and others 
ectoderm with small yolk-granules. In this way a skin of small 
cells is, as it were, cut out from the surface of the large cells 
for a considerable distance below the equator of the egg. 
The edge of this advancing skin represents the point x in 
Amphioxus. The cavity of the archenteron appears first as 
a split within the region of undoubted endoderm 
cells. (This is beautifully and clearly shown in the figures 
which Bles gives of the development of Xenopus [5].) Then 
later the endoderm cells wander inwards in virtue of an 
altered cyto-taxis, the arclienteric cavity simultaneously 
enlarges, and the segmentation cavity is obliterated. All 
question therefore of the roof of the archenteron being 
in-turned ectoderm is dis])osed of. The ventral lip of the 
blastopore makes its appearance later than the dorsal lip by 
a process of multiplication of cells, accompanied by differen- 
tiation of the daughters into ectoderm and endoderm, and it 
