POLYEMBRYONIC DEVELOPMENT IN TATUSIA 603 
mal cells are found just beyond the outer margin of the embry- 
onic ectoderm, in the space lying between the entoderm and the 
extraembryonic mesoderm, but such cells are undoubtedly given 
off from the marginal cells of the ectoderm. 
ORIGIN OF THE SECONDARY BUDS 
The formation of the secondary buds immediately follows the 
establishment of the primary diverticula, and three or four 
specimens in the*collection show the main steps in the process. 
However, it will be necessary to secure a closer series through 
this period of development before a detailed account can be 
given of the origin of the secondary buds. Each primary bud 
gives rise to two secondary buds, and consequently there are 
four secondary diverticula. Each secondary bud carries the 
rudiment or primordium of an embryo. The first step leading 
to the development of the secondary diverticula consists in the 
formation of two thickenings in the wall of each primary bud. 
One of these areas lies at the tip of the bud, while the other 
appears slightly to the left (as viewed from above) of the tip. 
The secondary buds then arise from these areas as blind diver- 
ticula, which extend down along the inner surface of the yolk- 
sacentoderm. Inspecimen No. 247 the beginning of the second- 
ary buds can be seen in the left-hand primary bud (fig. 1). At 
the point marked s is seen a slight protrusion which will form 
secondary bud No. III. 
The secondary buds soon become recognizable in surface views 
of living specimens, and appear as four blunt processes from the 
sides of. the ectodermal vesicle. Upon the upper surface of each 
bud an embryonic rudiment appears, in the form of a white, 
opaque spot. It is somewhat difficult to make out the exact 
limits of the different parts of the ectodermal structures, owing 
to the fact that the entoderm (and the chorionic ectoderm, 
if still present) tend to obscure the view. This difficulty was 
obviated by making an outline reconstruction from the series 
of sections of the blastocyst. 
JOURNAL OF MORPHOLOGY, VOL. 24, NO. 4 
