44 BIOLOGY OF THE LABORATORY MOUSE 



A beginning of this process can be seen in six somite embryos (Fig. i5), and 

 in ten somite embryos the tail fold is well developed (Figs. 26 A and 27). 

 The process is strictly comparable to the formation of the head fold except 

 for one interesting difference; whereas the head fold lies entirely within the 

 amniotic cavity, the tail fold lies only partly within it. The ventral surface 

 of the tail fold is in the exocoelom. This is because in its growth away from 

 the yolk-sac it pushes the base of the allantois and the adjacent margin of the 

 amnion ahead of it. The amnion remains attached to its caudal and lateral 

 walls, and only its dorsal surface is within the amniotic cavity (Fig. 24). 



The turning of the embryo. — Almost immediately after the seven somite 

 stage the embryo begins a process of turning which results in a reversal of 

 the curvature of the whole trunk region. Thus instead of being S-shaped 

 the embryo becomes C-shaped with the ventral surface everywhere on the 

 inside of the C. The turning begins in the head and tail folds, and consists 

 of a rotation of each along its long axis, or in other words, on axes parallel to 

 the fore- and hind-guts (Figs. 24-28). Viewing each fold from its cephahc 

 toward its caudal end, the direction of rotation is clockwise in each case. 

 Of course, both folds cannot be viewed in this direction from any one point, 

 because of the curvature of the embryo. Viewed from the mesometrial 

 pole, in sections the turning of the head fold appears to be clockwise, of the 

 tail fold counter-clockwise (Fig. 24). 



At first the turning is confined to the head and tail folds; the mid-trunk 

 region, still firmly attached to the yolk-sac, remains in its original position. 

 At about 8^^ days, and at about the eleven or twelve somite stage, the mid- 

 trunk region turns also. The process is sudden. Transverse sections of the 

 trunk region at about this period show it to be either turned or not turned 

 (Figs. 19C and 20A). It is quite possible that after the growth of the head 

 and tail folds reduces sufficiently the attachment of the trunk region to the 

 yolk-sac, this region snaps over like a spring whose tension has come to 

 exceed the forces holding it. Some time elapses after the turning of the mid- 

 trunk before the head and tail regions complete their rotation, which even- 

 tually amounts to a full 180°. Essentially, however, by about 9 days the 

 embryo has become concave towards the ventral surface (Figs. 26B and C). 



The mid -gut. — The turning of the mid-trunk region automatically results 

 in the formation of the mid-gut. Prior to turning, the two sheets of embry- 

 onic splanchnopleure in the mid-trunk region extend straight out from the 

 sides of the embryo, forming a virtually plain surface (Fig. 19C). There is 

 thus no indication of a mid-gut. When the mid-trunk region turns suddenly 

 towards its left side, the two sheets are pulled after it, forming between them 



