1886, J | Embryology. 663 
With regard to the retention of the primitive union of the cells 
of the ovum until this stage, nothing of the sort has been described 
_ before, except by Sedgwick in Peripatus. The most important 
part, it seems to me, is not the connection of cell to cell, but of 
layer to layer by means of processes of the cells. 
About the middle of the fourth day several of the hypoblast 
cells approach the epiblast in the middle line of what will even- 
7 tually be the ventral surface of the embryo. This is the begin- 
= ningofa mesoblastic keel such as Balfour described for Agalena. 
The epiblast cells in the ventral middle line, after altering their 
shape, increase by division and take a considerable share in the 
formation of the keel. The hypoblast cells below them also 
increase, and on the fifth day the mesoblastic keel is complete, in 
the formation of which both epiblast and hypoblast have taken 
F part. The keel is still present on the sixth day, but the cells 
composing it are becoming elongated in a plane parallel to its 
| surface. They then spread out to form two definite splanchnic 
and somatic layers of the mesoblast below the epiblast. These 
two layers are connected. The keel disappears on the seventh 
and. eighth days, and on the ventral surface the epiblast cells 
assume a columnar form, thus giving rise to the ventral plate. 
_ _ The mesoblast now becomes thicker on each side of the median 
line, both layers being concerned in this thickening, where they 
become indistinguishable. Outside the thickenings, that is, 
farther away from the middle line, the two layers are closely 
ie that of the future segments of the body. There are at first 
“gat somites, corresponding with the eight segments of the 
embryo when hatched. These somites are at first solid ; after- 
wards a cavity appears in them. 
FRR of the epiblast near one end of the ventral surface. Sho: 
s ae formation of the stomodaum the proctodzeum appears as 
_ “shallow somewhat wide invagination of the other end of the 
_ Ventral surface. 
44 
___ the body segments now become more apparent, each being — 
tly on the ninth day the stomodzeum is formed as an invagi- 
a 
