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 arc of approximately 

 80 degrees on the circumference. These points can be made out 

 in specimen No. 247, which will now be described. 



