936 



OF GENERATION EMBRYONIC DEVELOPMENT. 



FIG. 341. 



A. Heart of the Embryo of a Rabbit seen from 

 before: ta, truneus arteriosus ; /, left ventricle; 

 r, right ventricle; a, auricle ; v, venous sinus. 



B. The same heart seen from behind : a, vense 

 omphalo-mesaraicse ; d, right auricle ; e, Imlbus 

 aortte; /, the six aortic arches; e, atrium; b, 

 auriculae. 



tached, float in the newly formed cavity as the first blood-corpuscles, whilst 



the outer remain to constitute its walls. 

 For a long time after it has distinctly 

 commenced pulsating, and is obviously 

 exerting a contractile force, its walls 

 retain the cellular character, and only 

 become muscular by a progressive his- 

 tological transformation. The first ap- 

 pearance of the Heart in the Chick is 

 at about the 27th hour ; but the time 

 of its formation in Mammalia has not 

 been distinctly ascertained. In its 

 earliest form, it has the same simple 

 character which is presented by the 

 central impelling cavity of the lower 

 In vertebra ta ; being a mere prolonged 

 canal, which at its posterior extremity 

 receives the veins, and at its anterior 

 sends forth the arteries. About the 



15th-18th day in the Human Embryo it becomes doubly bent upon itself 

 (Plate II, Fig. 13, d, and Fig. 341, A, B), one loop corresponding to the arte- 

 rial, the other to the venous portion. After this, two slight enlargements 

 (b) are observed in the venous bend ; and the arterial bend separates into 

 two parts by a long line of division. The two enlargements represent the 

 auricles, and receive the veme omphalo-mesaraicse (a), which run outwards 

 in the folds of the Splanchnopleure. Above, they open into the atrium (c), 

 which leads to the right ventricle (d~), and this again into the bulbus aortse. 

 The circulation is at first carried on exactly upon the plan which is perma- 

 nently exhibited by Fishes. The Aorta subdivides on either side of the 

 neck into three or four arches (Figs. 340, 346, e, e', ee"), which are separated 

 by fissures much resembling those forming the entrances to the gill-cavities 

 of Cartilaginous Fishes; and these arches reunite to form the descending 

 aorta, which transmits branches to all parts of the body. The vessels as well 

 as the blood-corpuscles proceed from processes thrown out by mesoblastic 

 cells, which unite to form a protoplasmic network containing nuclei at the 

 point from which the processes started. The nuclei multiply and form nodal 

 points, and those occupying the centre of these nodal points become colored, 

 and constitute the corpuscles, whilst the outer ones remain imbedded in 

 granular protoplasm aud'form the vessel-wall, and thus a network of vessels 

 containing blood-corpuscles is produced (Foster and Balfour). Such is the 

 first phase or aspect of the Circulating Apparatus, which is common to all 

 Vertebrata during the earliest period of their development, and which may, 

 therefore, be considered as its most general form. It remains permanent in 

 the class of Fishes; and in them the vascular system undergoes further de- 

 velopment on the same type, a number of minute tufts being sent forth from 

 each of the arches, which enter the filaments of the gills, and are thus sub- 

 servient to the aeration of the blood. In higher Vertebrata, however, the 

 plan of the circulation is afterwards entirely changed, as will be presently 

 described, by the formation of new cavities in the heart, and by the produc- 

 tion of new vessels; it is incorrect, therefore, to speak of the vascular arches 

 in their necks as branchial arches, since no branchiae or gills are ever devel- 

 oped from them. The clefts between them may be very distinctly seen in 

 the Human Foetus towards the end of the first mouth; during the second, 

 they usually close up and disappear. 



779. With the evolution of a Circulating apparatus, adapted to absorb 



