784 



DEVELOPMENT OF THE VASCULAR SYSTEM. 



penetrating into the mesoblast only on the twelfth day in the chick. 

 The pulmonary arteries are developed in mammals in connection Avith 

 the fifth branchial arch of the left side, but the manner in which they 

 become connected with the vessels formed in the lung-substance, and the 

 manner in which a union is established between the pulmonary veins 

 and the left auricle have not yet been ascertained. 



DEVELOPMENT OF THE HEART AND BLOOD-VESSELS. 



In the account of the general phenomena of development the esta- 

 blishment of the first circulation of blood, by the simultaneous formation 

 of the simple heart and of the first blood-vessels and blood in the body 

 of the embryo and in the vascular area of the blastoderm, has already 

 been described, and in the General Anatomy (p. 180) an account has 



Fig. 583. 



Kg. 58S. — Outlines of the anterior 



HALF OF THE EjIBRYO ChICK A'IEWED 

 FROM BELOW, SHOWING THE HeART IN 

 ITS EARLIEST STAGES OF FORMATION 



(after Kemak). "» 



A, Embryo of ab(jut 20 to 30 lioxirs ; 

 B, of about 36 to 40 hours ; a, anterior 

 cerebi'al vesicle ; h, j)roto-vertebral seg- 

 ments ; c, amniotic fold ; 1, 1, primitive 

 omphalo-mesenteric veins entering tlie 

 Iieart posteriorly ; 2, their union in the 

 auricle of the heart ; 3, the miLldle part 

 of the tube corresponding to the ven- 

 tricle ; 4 (in B) the arterial bulb. 



been given of the histological 

 changes occurring in the first 

 development of the blood-vessels 

 and blood. 



DEVELOPMENT OF THE HEART. 



Origin of the Heart — Simple Tubular Foi'in. — The heart takes 

 its origin in the form of an elongated sac or dilated tube in the substance 

 of a thickening of the splanehuo-pleure layer of the mesoblast, in the 

 ventral aspect of the cephalic portion of the primitive alimentary canal, 

 immediately in front of the fovea cardiaca. Doubts have existed as to 

 the exact mode of production of the cavity of the organ, but the observa- 

 tions of Aflanasieff and Klein, and especially those of Foster and 

 Balfour, appear to show that the substance in which the first rudiments 

 of the heart arise is produced by a thickening of the lower wall of the 

 mesoblastic layer of the primitive intestine, and that the cavity is 

 formed by a solution of continuity or liquefaction of tliis substance in 

 such a manner that, while the outer cells constitute the foundation of 

 the commencing fibrous walls,, a deep set of cells very soon or from the 

 first arrange themselves in the form of an endo-vascular lining of the 

 cavity. The oi-gan has at first the form of an elongated sac or dilated 

 tube of symmetrical shape, widening out behind into two lateral 

 orifices, each of which is connected with an omphalo-mesenteric vein of 

 its own side bringing the nascent blood back from the vascular area, 

 while the anterior part of the rudimental heart leads into two arterial 



