DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 3895 
parts of the axial portion of the embryonic area corresponds in 
length with the whole of one side of the epiblastic cylinder. 
It is at the proximal end of this portion that the mesoblast 
first appears, marking the posterior or caudal end of the em- 
bryonic area; therefore, for convenience of reference, I shall 
call this side the caudal side (CAS, fig. 15), although it 
includes a little more than the caudal half of the embryonic 
area. ‘The opposite side is the cephalic side (CHS) of the 
embryonic cylinder. 
As the long axis of the embryonic area lies in a plane at 
right angles to the long axis of the uterus until the eleventh 
day, when the twisting of the embryo commences, to obtain 
longitudinal sections of the embryonic area between the eighth 
and eleventh days it is necessary to cut the uterus into vertical 
transverse sections. 
Coronal sections of the uterus between the eighth and 
eleventh days contain transverse sections of the embryonic 
cylinder ; the cephalic caudal sides of the cylinder appear in 
each section. The section of the cephalic side represents part 
of the anterior portion of the embryonic area; the section 
through the caudal side part of the posterior portion of the 
embryonic area, except in the case of those sections which 
have passed through the distal end of the cylinder beyond the 
neurenteric canal (fig. 14 A), and have, therefore, cut the an- 
terior part of the embryonic area twice—once on the cephalic 
and once on the caudal side of the cylinder. It must also be 
remembered that transverse sections which pass through the 
embryonic cylinder on the proximal side of the neurenteric 
canal cut the anterior and posterior halves of the em- 
bryonic area at unequal distances from the central point of the 
area. 
Returning now to the consideration of the main changes of 
form which take place during the eighth and ninth days, it has 
been shown that the appearance of the mesoblast at the pos- 
terior end of the embryonic area (figs. 14 and 15, Pl. XXIV) 
gives rise to the tail amnion fold (AFT, figs. 14 and 15). The 
appearance of the mesoblast at the cephalic end of the em- 
