DEVELOPMENT OF MAMMALIAN GASTRULA. 279 



cord arises between the spinal tube and the intestinal tube ; 

 this is the notochord, or chorda (c&). It originates from 

 the central portion of the skin-fibrous layer, while the side 

 portions of the latter supply the true skin and the great 

 part of the flesh. This flesh-mass separates into the dorsal 

 muscles (Fig. 68, 69 mi) and the ventral muscles (6m). 



The separation of the four secondary germ-layers is 

 followed by a separation between the skin-fibrous layer (Jif) 

 and the intestinal-fibrous layer (df). Between the two, 

 a chink-like cavity, filled with fluid, arises ; this is the true 

 body-cavity (cceloma, Fig. 65-69 c). The intestinal tube lies 

 freely in this, being only supported along the length of the 

 notochord by a band of the intestinal-fibrous layer, which 

 afterwards extends into the mesentery (Fig. 68 g). Two 

 naiTow canals, filled with blood, form within the intestinal- 

 fibrous layer, and traverse the whole length of the intestine, 

 one passing underneath, and the other above ; these are the 

 first blood-vessels. The upper of the two is the dorsal 

 vessel (Fig. 69 a), the latter is the ventral vessel (vd) ; the 

 one afterwards gives rise to the aorta, the other to the 

 intestinal vein and the heart. 



Finally, the first rudiments of the urinary and sexual 

 glands make their appearance on either side of the in- 

 testinal tube and the notochord attached to the dorsal 

 wall of the body-cavity. The primitive kidneys (u) re- 

 semble two narrow canals, traversing the body, parallel 

 to the notochord, opening at the anterior end into the 

 body-cavity, and at the posterior end through the outer 

 skin (or in the last chamber of the intestine). They 

 probably originally arose as skin-glands, formed by an 

 inversion of the skin-sensory layer (Fig. 66-68 u). In 



