600 J. T. PATTERSON 
cyst, appears in the formation of two diverticula from the ecto- 
dermal vesicle. In the preliminary paper these diverticula were 
termed the ‘primary buds,’ and I shall continue so to designate 
them. The buds appear on the opposite sides of the vesicle, 
and, with respect to the orientation of the blastocyst within the 
uterus, on the right and left sides, respectively, that is, on the 
sides of the vesicle that face the openings of the fallopian tubes. 
In accordance with the statement made above, the right primary 
bud faces the left fallopian tube opening, and similarly the left 
primary bud faces the right opening. 
The primary buds do not develop for some time after the 
completion of the ectodermal vesicle, although their appearance 
is anticipated soon after this period by certain easily detectable 
changes in the walls of the vesicle. It will be recalled that 
immediately after the ectodermal sphere has become transformed 
into a vesicle, that portion of the wall of the vesicle which is 
turned toward the free pole of the blastocyst is of a relatively 
uniform thickness (fig. 51). Very shortly thereafter one can 
detect a tendency in this region of the wall to become less thick 
(figs. 55-58). The thinning out may be due in part to an in- 
crease in size of the vesicle by the accumulation of fluid within 
its cavity, but undoubtedly in the main it is brought about 
through the shifting of cells from here to the lateral portions of 
the wall, for these show an increase in thickness (fig. 59). 
The shifting of the cells from the pole of the vesicle results in 
the formation of a thickened zone adjoining the thin or endo- 
thelial-like portion of the ectodermal vesicle (figs. 59, 61). The 
zone is not uniformly thick, but is thickest at the two regions 
corresponding respectively to the right and left sides of the vesi- 
cle. One can therefore correctly speak of these thickened areas 
as lateral plates. 
The primary buds arise from these lateral plates, and appear 
as two broad, blunt processes protruding from the sides of the 
ectodermal vesicle (fig. 21). Each bud involves the greater por- 
tion of the side of the vesicle, covering an are of approximately 
80 degrees on the circumference. These points can be made out 
in specimen No. 247, which will now be described. 
