128 



THE DOG IN HEALTH. 



complete coitus and mutually desired, a second is super- 

 fluous ; but of this one can never be sure. 



The mode of coitus in the dog is characteristic, owing 

 to the peculiar structure of the penis, the action of the 

 female's vagina, etc. 



FIG. 8. EMBRYO OP DOG, TWENTY-FIVE DAYS OLD, OPENED ON VENTRAL SIDE. 

 CHEST AND VENTRAL WALLS HAVE BEEN REMOVED. 



a, nose-pits ; 6, eyes ; c, under-jaw (first gill-arch); rf, second gill-arch ; e, /,<;, ft, 

 heart (e, right, /, left auricle ; g, right, ft, left ventricle); i, aorta (origin of); 

 fcfc, liver (in the middle between the two lobes is the cut yelk- vein); Z, stomach ; 

 m, intestine ; n, yelk-sac ; o, primitive kidneys ; p, allantois ; g, fore-limbs ; 

 ft, hind-limbs. The crooked embryo has been stretched straight. (Haeckel, 

 after Bischoff.) 



The penis consists of two erectile portions. In the 

 anterior part of the organ there is a bone. The posterior 

 erectile region is the larger, and during copulation is spas- 



