340 THE EVOLUTION OF MAN. 



observed, these side-layers, which for a time were separated 

 from the primitive vertebral cords, afterwards again adhere 

 to the latter. While the inner portion of the side-layers 

 (belonging to the intestinal-fibrous layer) is thus forming 

 the external wall of the intestine, the outer portion of the 

 same layers (belonging to the skin-fibrous layer) grows in 

 a circle round the intestine, thus closing the body-cavity 

 (Fig. 100, p. 333). The edges of the ventral plates, as these 

 portions of the side-layers are called, grow toward each 

 other from all sides, continually narrowing the slit-like 

 ventral opening, from which the yelk-sac depends. Finally, 

 the latter is, in Mammals, completely separated from 

 the intestine by the closing of the ventral plates, while in 

 Birds and Reptiles it is taken into the intestine. This point 

 at which the ventral wall finally closes — the last point of 

 coalescence — is the ventral navel, the externally visible skin- 

 navel, which is commonly briefly called the navel. This 

 must be distinguished from the inner, or intestinal navel, 

 which is the point at which the intestinal canal closes, and 

 of which no trace can afterwards be found. With the 

 closing of the intestinal tube and of the ventral wall, 

 the double tubular form of the vertebrate body is com- 

 plete. 



A few words must still be said concerning the modifica- 

 tions which, while these processes are going on, take place 

 in the primitive kidneys and in the blood-vessels. The 

 primitive kidneys, which at first lie quite superficially just 

 below the outer skin (epidermis, Fig. 99, ung), soon penetrate 

 far into the interior in consequence of peculiar conditions of 

 growth (Figs. 95, 96, ung, pp. 317, 318) ; at last they lie very 

 far within, underneath the chorda dorsalis (Fig. 97, un, p. 318). 



