MAMMALIA. 



189 



seen in the lateral plates of mesoblast into a vertebral zone adjoining 

 the embryo and a more peripheral lateral zone ; and in the verte- 

 bral zone indications of two somites, about 0'37 mm. from the hinder 

 end of the embryo, become apparent. The foremost of these somites 

 marks the junction, or very nearly so, of the cephalic region and 

 trunk. The small size of the latter as compared with the former is 

 very striking, but is characteristic of Vertebrates generally. The 

 trunk gradually elongates relatively to the head, by the addition 

 behind of fresh somites. The embryo has not yet begun to be 

 folded off from the yolk-sack. In a slightly older embryo of nine 

 days there appears (Hensen, Kblliker) round the embryonic area 

 a delicate clear ring which is narrower in front than behind (fig. 

 144 A. ap). This ring is regarded by these authors as representing 

 the peripheral part of the area pellucida of Birds, which does not 



A. 



B. 



no 



I \ .Jill.!!""'! 



7,1. 



BJK] !*"* 



.iVpv 1 ' 

 "-'- \ : I 



W.M ; '".v i 



rt 



FIG. 144. EMBRYO RABBITS OF ABOUT NINE DAYS FROM THE DORSAL SIDE. 



(From Kolliker.) 



A. magnified 22 times, and B. 21 times. 



up. area pellucida; rf. medullary groove ; h'. medullary plate in the region of the 

 future fore-brain; It", medullary plate in the region of the future mid-brain; rh. fore- 

 brain; ab. optic vesicle; mh. mid-brain; Jih. and h'". hind-brain; uw. mesoblastic 

 somite; stz. vertebral zone; pz. lateral zone; hz. and h. heart; ph. pericardia! section 

 of body-cavity; ro. vitelliue vein; nf. amniou fold. 



