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 



mp 



uh 

 A 



ml? 



Fig. 105 Two cross sections through a Triton embryo. 



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



the primitive segments begin to be constricted off from the lateral plates. 



B, Cross section through the region of the trunk in which the neural tube is closed and the 



primitive segments have been formed. 



mf, Medullary folds ; mp, medullary plate ; n, neural tube ; ch, chorda ; ak, outer, ik, inner 

 germ-layer ; mJc 1 , parietal, m/fc 2 , visceral middle layer ; dh, intestinal cavity ; Ih, body-cavity 

 ush, cavity of primitive segment ; dz, yolk-cells. 



Toward the ventral side they insert themselves between the secondary 

 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 their lumina and furnish the transversely striped 



