648 



EMBRYOLOGY. 



heart-formation, however abeivant it at first sight appears, can be 



easily referred back to this. 



A single cardiac tube cannot be developed in the higher 



Vertebrates, because at the 

 time of its formation a head- 

 gut does not yet exist, but 

 only the fundament of it is 

 formed in the still flat ento- 

 derm. The parts which will 

 subsequently form the ventral 

 wall of the head-gut, and in 

 which the heart is developed, 

 are still two separated terri- 

 tories; they still lie at some 

 distance from the median 

 plane at the right and at 

 the left. If therefore it is 

 necessary for the heart to be 

 formed at this early period, 

 it must arise in the separated 

 regions, which by the process 

 of infolding are joined into 

 a single ventral tract. The 

 vessel must arise as two parts, 

 which, like the two intestinal 

 folds, subsequently fuse. 



Whether the heart is formed 

 in one way or the other, in 

 either case it has for a time 

 the form of a straight sac 

 lying ventral to the head -gut 

 and composed of two tubes one 

 within the other, which are 

 separated by a large space 

 assumably filled with a gela- 

 tinous matrix. The inner, 

 endothelial tube becomes the 

 endocardium ; the outer tube, 



which is derived from the visceral middle layer, furnishes the 



foundation for the myocardium and the pericardial membrane that 



immediately invests the surface of the heart. 



ap 



Fig. 302. Embryo Rabbit of the ninth day, seen 

 from the dorsal side, after Kiii.i.iKr.R. Mag- 

 nified 21 diameters. 



The axial (stem-) zone (stz) and the parietal zone 

 (j>z) are to be distinguished. In the former 8 

 pairs of primitive segments have been formed 

 at the side of the chorda and neural tube. 



ap, Area pellucida ; rf, dorsal groove; vh, fore 

 brain ; ab, optic vesicle ; mh, mid-brain; hh, 

 hind-brain ; uic, primitive segment ; stz, axial 

 zone ; pz, parietal zone ; h, heart ; ph, pericar- 

 dial part of the body-cavity ; rd, margin of the 

 anterior intestinal portal showing through the 

 overlying struct in t-> ; /. fuhl .f tin- nmninn ; 

 vo, vena onphaloiiMMnteiioft, 



