^ DEVELOPMENT OF MAMMALIAN GASTRULA. 279 



cord arises between the spinal tube and the intestinal tube ; 

 this is the notochord, or chorda {ch). It originates from 

 the central portion of the skin-fibrous laye^^ while the side 

 portions of the latter supply the true skin and the great. 

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

 muscles (Fig. 68, 69 rm) and the ventral muscles (bm). 



The separation of the four secondary germ-layers is 

 , followed by a separation between the skin-fibrous layer (A/) 

 \ and the intestinal-fibrous layer (c?/). Between the two, 

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

 I body-cavity (coeloma, 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 

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

 ; narrow canals, filled with blood, form within the intestinal- 

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

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

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

 I vessel (Fig. 69 «), the latter is the ventral vessel (yd); the 

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

 [I 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- 

 f semble two narrow canals, traversing the body, parallel 

 J 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-senscry layer (Fig. 66-68 u). In 



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