FORMATION OF THE ALLANTOIS AND YELK-SAC. 315 



Tery wide opening by which the intestinal cavity at first 

 communicates with the navel bladder, afterwards grows 

 continually narrower, and at last altogether disappears. The 

 navel, the little pit-like depression which appears in the 

 middle of the ventral wall of the developed Man, is the 

 place at which the remains of the germinal vesicle, the navel 

 bladder, once entered the intestinal cavity, and by which it 

 was connected with the intestine in the course of its evolu- 

 tion. (Cf. Figs. 14 and 15 on Plate V.) 



The formation of the navel takes place at the same time 

 as the closing of the outer ventral wall. The ventral wall 

 originates in exactly the same way as the dorsal wall ; 

 both are formed essentially from the skin-fibrous layer, 

 and are covered outwardly by the horn-plate, the peripheric 

 part of the skin-sensory layer. Both are formed by the 

 modification of the animal germ-layer into a double tube; 

 above, at the back, the vertebral canal, which encloses the 

 spinal tube, — below, at the abdomen, the wall of the body- 

 cavity, which encloses the intestinal tube (Fig. 93, p. 309). 



We will first notice the formation of the dorsal wall, 

 and then that of the ventral wall (Figs. 95-98). In the 

 centre of the dorsal surface of the embryo the spinal tube 

 (mr) lies, originally immediately below the horn-plate (h), 

 from the central part of which it has separated. But, at 

 a later period, the primitive vertebral plates (uw) grow 

 from the right and the left so as to penetrate between 

 these two originally connected parts (Figs. 97, 98). The 

 upper inner edges of the two primitive vertebral plates 

 wedge themselves in between the horn-plate and the spinal 

 tube, press these two apart, and finally coalesce between 



them in a suture corresponding with the central line of 

 23 



