374 THE EVOLUTION OF MAN. 



directly by digestion, from the intestinal wall, and conduct 

 it to the blood-current, are distinguishable as chyle-vessels, 

 or "milky juice vessels." While the chyle, or milky juice, 

 in consequence of the great amount of fat globules which 

 it contains, appears milk white, the real lymph is colour- 

 less. The chyle, as well as the lymph, contain the same 

 colourless amoeboid cells (Fig. 9, vol. i. p. 132), which are also 

 distributed in the blood as colourless blood-cells (corpuscles) ; 

 the latter contains, in addition, the much greater quantity 

 of red blood-cells (corpuscles), which gives the blood of 

 Skulled Animals its red colour. The distinction, common to 

 all Craniota, between lymph-vessels, chyle-vessels, and 

 blood-vessels, is to be regarded as the result of a division of 

 labour which took place between different portions of an 

 original unitary, primitive blood-vessel system (or hsemo- 

 lymph system). 



The heart, the central organ of the circulation of the 

 blood, which exists in all Craniota, also exhibits an advance 

 in structure, even in the Cyclostoma. The simple spindle- 

 shaped heart-pouch is separated into two divisions, or 

 chambers, which are divided by two valves (Plate XL 

 Fig. 16, hv, hk). The posterior division, the fore chamber 

 (atrium,, hv), absorbs the venous blood from the veins of 

 the body, and discharges it into the anterior division, the 

 chamber, or main chamber (ventriculus, hfy. From here it 

 is propelled by the gill-artery stem (the foremost section of 

 the ventral vessel) iiito the gills. 



In Primitive Fishes (Selachii), an arterial stalk (bulbus 

 arteriosus), separated by valves, originates, as a distinct 

 section, from the foremost end of the ventricle. It forms 

 the enlarged, hindmost end of the gill-artery stem (Fig, 



