230 



GENERAL GROWTH OF THE EMBRYO. 



The structure of the head and the formation of the heart at 

 this age are illustrated in fig. 145. The widely-open medullary 

 groove (rf) is shewn in the centre. Below it the hypoblast is 

 thickened to form the notochord dcf ; and at the sides are seen 

 the two tubes, which, on the folding-in of the fore-gut, give rise 

 to the unpaired heart. Each of these is formed of an outer mus- 

 cular tube of splanchnic mesoblast (a/i/i), not quite closed towards 

 the hypoblast, and an inner epithelioid layer (ik/i) ; and is placed 



A. 



B. 



FIG. 145. TRANSVERSE SECTION THROUGH THE HEAD OF A RABBIT OF THE SAME 

 AGE AS FIG. 144 B. (From Kolliker.) 



B. is a more highly magnified representation of part of A. 



rf. medullary groove ; mp. medullary plate ; rw. medullary fold ; h. epiblast ; 

 dd. hypoblast; dd' . notochordal thickening of hypoblast; sp. undivided mesoblast; 

 tip. somatic mesoblast ; dfp. splanchnic mesoblast ; ph. pericardial section of body 

 cavity; ahk. muscular wall of heart; ihh. epithelioid layer of heart; nies. lateral 

 undivided mesoblast ; s?v. fold of hypoblast which will form the ventral wall of the 

 pharynx ; sr. commencing throat. 



in a special section of the body cavity (//^), which afterwards 

 forms the pericardial cavity. 



Before the ninth day is completed great external changes are 

 usually effected. The medullary groove becomes closed for its 

 whole length with the exception of a small posterior portion. 

 The closure commences, as in Birds, in the region of the mid- 

 brain. Anteriorly the folding-off of the embryo proceeds so far 



