328 THE MAMMALIAN EMBRYO. [CHAP. 



medullary folds nearly meet behind and so cut ofif the 

 front portion of the primitive streak, which then ap- 

 pears as a projection in the hind end of the medullary 

 groove. At the hind end of the medullary groove 

 (mole) a deep pit perforates its floor and enters the 

 mass of mesoblast cells lying below. The pit is a rudi- 

 ment of the blastopore (described on p. 326) which has 

 been enclosed by the medullary folds. 



Henceforward the general course of development is 

 very similar to that in the chick and so will be only briefly 

 described. The special features in the development of 

 particular organs will be described later. In an embryo 

 rabbit, eight days after impregnation, the medullary 

 groove is about 1"80 mm. in length. At this stage a 

 division may be clearly seen in the lateral plates of 

 mesoblast into a vertebral zone adjoining the embryo 

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

 bra] 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 gene- 

 rally. 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-sac. 



In a slightly older embryo of nine days there appears 

 (Hensen, KoUiker) round the embryonic area a delicate 

 clear ring which is narrower in front than behind (Fig. 

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

 representing the peripheral part of the area pellucida of 



