TO THE LOWER ANIMALS 



249 



of this chamber appears a solid cellular cord, the so-called 

 ' notochord.' One end of the inclosed cavity dilates to 

 form the head (Fig. 13, B), the other remains narrow, and 

 eventually becomes the tail ; the side walls of the body 

 are fashioned out of the downward continuation of the 

 walls of the groove ; and from them, by and bye, grow out 

 little buds which, by degrees, assume the shape of limbs. 

 Watching the fashioning process stage by stage, one is 

 forcibly reminded of the modeller in clay. Every part, 

 every organ, is at first, as it were, pinched up rudely, and 



FIG. 13. A. Earliest rudiment of the Dog. B. Rudiment further 

 advanced, showing the foundations of the head, tail, and vertebral 

 column. C. The very young puppy, with attached ends of the 

 yelk-sac and allantois, and invested in the amnion. 



sketched out in the rough ; then shaped more accurately ; 

 and only, at last, receives the touches which stamp its final 

 character. 



Thus, at length, the young puppy assumes such a form 

 as is shown in Fig. 13, G. In this condition it has a dis- 

 proportionately large head, as dissimilar to that of a dog 

 as the bud-like limbs are unlike his legs. 



The remains of the yelk, which have not yet been applied 

 to the nutrition and growth of the young animal, are 

 contained in a sac attached to the rudimentary intestine, 

 and termed the yelk-sac, or * umbilical vesicle.' Two 

 membranous bags, intended to subserve respectively the 



