CHAPTER VI 



VASCULAE SYSTEM 



As has already been indicated the vascular system of the animal 

 body consists- of strands of highly specialized mesenchyme — the 

 cells (corpuscles) along the axes of the strands being detached from 

 one another and floating freely in a fluid intercellular substance 

 (plasma), while the superficial cells are united together to form the 

 walls of tubular channels — the. vessels. The vessel walls are 

 provided with a coating of muscle-fibres and this muscular coat 

 becomes greatly thickened and specialized at one or more points to 

 form hearts which serve as pumps to force the blood through the 

 system of vessels. 



The fundamental plan of the Vertebrate vascular system appears 

 to have been like that of an Annelid worm, with two main longi- 

 tudinal blood-vessels, situated respectively one on the neural side of 

 the alimentary canal and one on the side opposite to this, connected 

 together by a series of half-hoop shaped vessels encircling the 

 alimentary canal laterally. Tn the Vertebrate the longitudinal 

 vessel on the neural side of the alimentary canal is the dorsal 

 aorta and in it the blood runs in a tailward direction. The longi- 

 tudinal vessel on the other (ventral) .side of the alimentary canal 

 develops the heart on its course : its precardiac portion is the 

 ventral aorta, its postcardiac the subintestinal vein. In this 

 ventral vessel the blood passes in a headward direction. Half-hoop 

 shaped vessels lying in front of the heart and connecting ventral 

 aorta and dorsal aorta are the aortic arches. 



Origin of the Heart and Vessels in the Holoblastic 

 Vertebrates. — Amongst holoblastic Vertebrates the first steps in the 

 development of the vessels have been investigated in the Newt 

 {Triton) by Mollier (1906) and his account will here be followed. 



In an embryo with six mesoderm segments the lateral sheets of 

 mesoderm have met ventrally except in the region of the liver where 

 they terminate in a free edge. This free edge is thickened and the 

 thickening extends back along the mid-ventral line towards the 

 cloaca as the rudiment of the subintestinal vein — the entire 

 thickening having thus a Y-shape (Fig. 173, A). 



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