
No. 427.] THE EGG OF BUFO LENTIGINOSUS. 537 
Zellenreihen bilden werden d. h. sie verwandeln sich in Ento- 
derm, wahrend die anderen zwei Zellenreihen der Blastula auch 
in ihrer Riickwindung beisammen bleiben und eine selbstandige 
Schichte, das Mesoderm, bilden." 
According to Lwoff, “die Einwanderung der Ektoderm- 
zellen und die Verschiebung der Entodermzellen zwei ver- 
schiedene und von einander unabhängige Vorgänge sind. 
Die Einwanderung von Ektodermzellen beginnt an der Stelle 
die das Hinterende des Embryo markirt ; die Verschiebung der 
Entodermzellen beginnt im Gegentheil im vorderen Theile. 
Wie die weiteren Stadien lehren, bildet das Hineinwachsen 
der Ektodermzellen die zusammenhangende ektoblastogene 
Anlage der Chorda und des Mesoderms; die Verschiebung 
der Entodermzellen führt zur Bildung der Darmhohle, die 
infolge des Auseinanderweichens der Entodermzellen ent- 
steht." 
Brauer's investigations show that there is a turning under 
of the outer surface cells, or “animal cells," at the posterior 
end of the germ disk and that by the forward growth of these 
cells under the upper animal cells a blind sac is formed. This. 
blind sac becomes the posterior part of the archenteron, and at 
first its dorsal wall is composed of the animal cells invaginated 
from the surface. In subsequent development a layer of vege- 
tative or yolk cells grows up from either side of the archenteron 
and forms a layer of endoderm beneath the invaginated animal 
cells which later become the mesoderm. 
The origin of the mesoderm in the frog's egg has been 
described by Morgan (16) as follows: ** The cells that are to 
form the mesodermal layer are present at the time when the 
dorsal lip of the blastopore has first appeared, and even just 
prior to that time.” The innermost of the cells forming a 
ring around the equatorial region of the egg where the black 
and the white cells meet are the cells that later become the 
mesoderm. “These cells are carried up to the median dorsal 
line of the embryo by the closure of the blastopore. They will 
then be found forming a layer or sheet of cells that separates 
itself on the outer side from the thick layer of small ectoder- 
mal cells (that has been simultaneously lifted up) and that is 
