DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 403 
caudal (CA S.) and lateral aspects of the cylinder; whilst on 
the cephalic side (CE S.), at this period, they show no distinct 
evidence of activity. 
In mice embryos at the commencement of the ninth day of 
gestation many of the succeeding stages of mesoblastic forma- 
tion may be readily followed. They are shown in the series 
of figs. 15 to 15 H, which represent sections of different mice 
ova from one uterus. The differences between them are there- 
fore small, but some of the ova are evidently a little more 
advanced than others. 
Longitudinal sections of the least developed of the series are 
shown in figs. 15,15 C, and 15 D, Pl. XXIV. The neurenteric 
canal is still present (figs. 15 and 15 D) at the centre of the 
embryonic area, which is about 544 4 long. The upper aper- 
ture of the canal is not distinct, but its lower opening is 
moderately well marked, though the canal is only an exceed- 
ingly narrow channel. In front of its lower opening the 
epiblast and hypoblast are fused, and for a short distance 
towards the cephalic extremity of the embryonic area the 
hypoblast is thickened, and contains a number of irregularly 
placed nuclei. 
Behind the neurenteric canal, in the anterior region of the 
primitive streak, the epiblast and hypoblast are not separable, 
though a series of flattened cells on the inferior surface of the 
streak suggest a thin and expanded hypoblastic layer. This 
suggestion is, however, not confirmed by transverse sections 
of a slightly more advanced embryo (figs. 15 F and 15 G, Pl. 
XXV), which show that in the region of the anterior part of 
the primitive streak the epiblast and hypoblast are fused. In 
longitudinal sections (fig. 15) the hypoblast beneath the 
posterior third of the primitive streak appears as a distinct 
layer, and transverse sections of the same region show that 
it is separated from the mesoblast by a cleft-like space 
(fig. 15 H). 
The extension of the mesoblast round the sides of the 
cylinder is much more advanced than in the preceding stage, 
for at the ninth section from the distal end of the cylinder 
