THE THIRD DAY. 

 Fia. 43. 



Transvbrsb section through the trunk of a Duck Embryo 

 with about twenty-four mesoblastic somites. 



ami. amnion ; so. somatopleure ; sp. splanchnopleure ; wd. "Wolffian 

 duct ; St. segmental tube ; ca.v. cardinal vein ; ins. miiscle^ 

 plate ; sp.g. spinal ganglion; sp.c. spinal cord; ch. notoohord; 

 ao. aorta ; hy. hypoblast. 



united with the side of the spinal cord at a very con- 

 siderable distance from its original point of origin. It is 

 moreover attached, not by its extremity, but by its side. 

 The above points, which are much more easily 

 studied in some of the lower vertebrate forms than in 

 Birds, are illustrated by the subjoined section of an 

 Elasmobranch embryo, Fig. 45. 



