164 



EMBRYOLOGY. 



backwards. At the same time the primitive segments (fig. 104} 

 gradually spread out both dorsally and ventrally, while their cells 

 increase in number and become altered in form. They grow upward 

 more and more at the side of the neural tube, which has meanwhile 

 detached itself completely from its matrix, the outer germ-layer. 



mf 



Fig. 106 Two 



igh a Triton embryo. 



nJP 



A, Cross Motion through the region of the trunk in which the neural tube U not yet closed an 



the primitive tegmenU begin to be constricted off from the lateral plate*. 



B, Cnm Motion through the region of the trunk in which the neural tube is closed and the 



primitive MgmenU have been formed. 



/, Medullary folds; mp, medullary plate; *, neural tube; eft, chorda; air, outer. ,t. inner 

 germ-layer ; m* 1 , parietal, M**, visceral middle layer ; <tt, intestinal cavity ; Ik, body-cavity 

 tuA, cavity of primitive segment ; d*, yolk-cells. 



T->\\;iril tin- ventral M<h- they insert themselves between the second:) ry 

 intestine and the outer germ-layer 



Finally, it might be further mentioned here that at a still later 

 stage, as is to be seen on the right side of fig. 104, the dorsal portions 

 of the primitive segment are constricted off from the ventral. The 

 former lose thrir luraina and furnish th rsely striped 



