4.02 ARTHUR ROBINSON. 
epiblastic cylinder—that is, forwards towards the anterior end 
of the cephalic side (CE S.) and backwards towards the 
posterior end of the embryonic area on the caudal side 
(CA S.), it becomes evident that the thickening of the hypo- 
blast gradually extends further and further round the circum- 
ference of the cylinder (see figs. 14 D, the fifteenth section, 
and 14 E£, the thirty-third section) ; and it is further noticeable 
that at the thirty-third section spaces begin to appear in the 
thickened hypoblastic layer. These spaces gradually assume 
a more definite arrangement (fig. 14 F) at the fortieth section, 
and at the fifty-second section (fig. 14 G) they have fused, 
separating a third layer, the mesoblast, from the thickened 
hypoblast, except to the right of the section on the cephalic 
side, to which the thickening of the hypoblast has now ex- 
tended. At the fifty-eighth section (fig. 14 H), which passes 
in the direction of the line A in fig. 14, the massive meso- 
blastic plates are entirely separated from the hypoblast, but 
they remain connected on the caudal side with the primitive 
streak. In this region the greater part of the hypoblast 
appears to be formed of large columnar cells, which contain 
large rounded nuclei at their bases ; but in the middle line on 
the cephalic side the hypoblast is still a homogeneous proto- 
plasmic layer which contains a double row of nuclei. 
At the sixtieth section, 9 u further forward on the cephalic 
side, the apices of the mesoblastic plates are again fused with 
the hypoblast, from which they do not again separate. Beyond 
the proximal end of the epiblastic cylinder the mesoblast 
forms a continuous sheet, which intervenes between the tro- 
phoblast and hypoblast in the middle line on the caudal side ; 
and as the sections pass further forward on the cephalic, and 
further backward on the caudal side, the margins of the meso- 
blastic plate recede gradually from the cephalic towards the 
caudal side, and thirty-six micromillimetres beyond the 
posterior end of the primitive streak the mesoblast is only 
found on the caudal side (fig. 14 J, Pl. XXIV), where it forms 
a thin layer not distinctly separated from the hypoblast, in 
which the nuclei are in an active state of proliferation, on the 
