

No. 427.] THE EGG OF BUFO LENTIGINOSUS. 533 
between marginally pigmented yolk cells, then the pigment 
must at once almost entirely disappear on one side of the split 
and not on the other. 












NEAN, PE CO YN 
Boge Tu E 

x : d = 
The pigmentation at CES OLYAN Ete, 
INE 
the anterior end of the SESE he 
archenteron in the egg COREE ERM 
of Bufo is not confined AETIA 
* S.C. — (ees SECO 
to * a double row of yolk € AY ees 
cells" anterior to the CI ger SN 
à TaT a 
actual cleft. Well Ay] SD 

marked lines of pig- 
ment may extend out in 
any direction, even at 
right angles to the line 
of advance of the arch- 
enteron, as seen in Fig. 
I3. From thetime that 
the blastopore is first 
formed, the pigment in 
the cells of the dorsal 
wall of the archenteron 
is always collected 
around the cell walls, 



s 
A 
INN 
S 
x1 
E 


Ce 
Bex 
me 
E 
NS 
dl G rt of a median sagittal section through the 
cleus. Rhumbler (22) blastopore at the stage of Fig. 3. Y., yolk plug. Other 
considers this phenom- lettering as in previous figures. 
enon to be due to the mechanical effect of pressure. The 
suggestion has been offered by Jordan “that the pigment marks 
physiological activity, and that the less heavily pigmented cells 
of the ventral wall of the archenteron owe their relative lack 
of pigment to more sluggish metabolism attendant upon less 
rapid cell division.” It is certainly true that the large yolk 
cells in the egg of Bufo divide less frequently than the cells 
in the upper hemisphere, but there is no evidence that the 
deeply pigmented cells of the outer surface of the upper 
hemisphere or of the dorsal wall of the archenteron divide 
more rapidly than the cells that are found between them. If, 
therefore, Jordan’s suggestion is correct, some kind of 
