400 ARTHUR ROBINSON. 
darkly staining epiblastic and faintly staining hypoblastic 
protoplasm. But though there is no active proliferation of 
the hypoblast in the first stages of its fusion with the epiblast 
in the posterior portion of the embryonic area, such prolifera- 
‘tion does not long remain absent, for at the posterior end of 
the area of fusion, which has an extent of 67:5 yw, there are 
distinct signs of activity in the nuclei of the hypoblast. 
The position of the thickened area of epiblast, between the 
neurenteric canal and the proximal end of the epiblast (fig. 
13), that is, between the neurenteric canal and the posterior 
end of the embryonic area in the mid-axial line, indicates that 
it is the rudiment of the primitive streak. 
The groove which appears upon the surface of the streak is, 
therefore, the primitive groove. In an embryonic area about 
532 uw long, the groove is only about 17 » long. It is deepest 
in front at the neurenteric canal. In an embryonic area 540 
uw long the primitive groove is 190 » long; it is present from 
the neurenteric canal to within 5 u of the posterior end of the 
embryonic area, therefore it extends backwards, and as the 
- groove extends, the thickened epiblast of the primitive streak 
becomes thinned out beneath it for a time (compare fig. 13 C, 
with fig. 13 D, Pl. XXIV). 
When the epiblast tube has attained a length of 288 u in 
the rat, and the germinal area is therefore about 576 mu long 
(fig. 14), the general relations are similar to those described 
when the germinal area was 532 mw long, except that from 
the posterior end of the primitive streak a nodular mass of 
mesoblast projects between the trophoblast and hypoblast. 
Selenka (44) has termed this mass the “ Allantoisknospe.” It 
will, however, be shown, in connection with the description 
of formation of the extra-embryonic celom, that the true 
allantoic projection does not appear until a considerably later 
period—that is, after the formation of the caudal and cephalic 
portions of the extra-embryonic ccelom. 
The neurenteric canal is still present, but is just as indis- 
tinctly marked as in the preceding stage (N C, fig. 14 A, Pl. 
XXIV), except at its lower part where a distinct space is visible. 
