THE DEVELOPMENT OF THE EMBRYO ITSELF. HI 



The allantoic vein is finally represented in its degenerated 

 form as a solid cord (round ligament), the entire venous sup- 

 ply of the liver being derived from the portal vein. 



The development of the heart has already been traced in the 

 fowl up to a certain point. In the mammal its origin and early 

 progress are similar and its further history may be gathered 

 from the following series of representations. 



In the fowl the heart shows the commencement of a division 

 into a right and left half on the third day, and about the fourth 

 week in man, from which fact alone some idea may be gained 

 as to the relative rate of development. The division is effected 

 by the outgrowth of a septum from the ventral wall, which rap- 



Fio. 118. 



Fis. 117. 



Fia. 117.— Development of the heart in the hnman embryo, from the fourth to the 

 sixth week. A. Embryo of four weeks (KOlliker, after Coste). B, anterior, C, 

 posterior views of the heart of an embryo of six weeks (KOlliker, after Ecker). 

 a. upper limit of buccal cavity; c, buccal cavity; b, lies between the ventral ends 

 of second and third branchial arches; d, buds of upper limbs; e, liver; /, intes- 

 tine; 1, superior vena cava; 1', left superior vena cava; 1", opening of inferior 

 vena cava; 2, 8', right and left auricles; 3, 3', right and left ventricles; 4, aortic 

 bulb. 



FiQ. 118.— Human embryo of about tliree weeks (Allen Thomson), uv, yelk-sac; al, 

 allantois; am, amnion; ae, anterior extremity; pe, posterior extremity. 



idly reaches the dor.sal side, when the double ventricle thus 

 formed communicate by a right and a left auriculo-ventricular 

 opening with the large and as yet undivided auricle. Later an 

 incomplete septum forms similar divisions in the auricle ; the 

 aperture (foramen ovale) left by the imperfect growth of this 

 wall persisting throughout fcetal life. 



The Eustachian valve arises on the dorsal wall of the right 

 auricle, between the vena cava inferior and the right and left 



