; 1890]  Concrescence Theory of the Vertebrate Embryo. 703 
The two-layered stage is found in the rabbit about seven, in 
the sheep about thirteen days after coitus. The dimensions for 
the sheep are about 4 mm. for the greatest diameter and 2~3 mm. 
for the lesser diameter. 
The two layers form each a closed sack; the embryonic 
shield is well marked asa round spot less translucent than the 
walls elsewhere. The outer layer has everywhere a distinctly 
epithelial character; in the region of the shield its cells are col- 
umnar with spherical nuclei; in the rabbit the cells are low, and 
the nuclei lie nearly at one level (for a good figure see Heape, 23, 
Pl. xxı, Fig. 49); in the sheep the cells are taller, and the nuclei 
are at various levels; in the mole and in various rodents there 
are several layers apparently, but perhaps in them also the epi- 
thelium is columnar, as it certainly is later; at the edge of the 
shield there is an abrupt change to a very thin layer, with widely : 
expanded cells; consequently in the region of the shield the nu- 
clei are close set, while outside the shield they are wider apart. 
The change at the edge of the shield is at first less abrupt, but 
at the present stage is very marked. A similar difference exists 
in the inner layer: although its cells are very much thinned out 
everywhere, yet the layer is slightly thicker in the region of the 
shield; the nuclei of the inner layer are everywhere somewhat 
flattened, arid they are larger and farther apart than the 
nuclei of the outer layer, a difference which is very obvious in 
surface views, hoth during this and the next following stages. The 
inner layer has an epithelial character in the region of the shield, 
but further away the cells move apart, and being connected by 
processes resemble embryonic connective tissue (Bonnet, 70, 192; 
Hensen, 24, Figs. 15 and 11B on Taf vii; E. Van Beneden, 2). 
The relations are illustrated by the accompanying Fig. 17, 
representing the shield in the sheep at thirteen days, and of a 
vesicle measuring 4 mm. by 2 mm.; at the left of the figure the 
layers are folded back over the shield. 
The next changes which occur are principally those of growth, 
both of the vesicle as a whole and of the embryonic shield; 
which also begins to arch up; the vesicle and shield both become 
oval; usually the oval shield lies lengthwise, but in the deer, as 
