Comparative Development. 41 



is, chyme is the only and permanent blood in the polyps, jelly-fishes, 

 some mollusks and some worms. A second and improved modification 

 of the blood is produced in the vertebrates by the mixture with it of the 

 ferments called pancreatic juice, and bile. In this form in man it is 

 called chyle. It is taken from the intestine by the lacteals, and carried 

 by the thoracic duct and poured into the left subclavian vein, where it 

 enters the general circulation and is changed to red blood in the passage 

 through the lungs. The chyle or white blood is the permanent blood of 

 insects, crabs and lobsters, most mollusks and some worms. 



Lymph is a colorless fluid composed of nutritive matter of nearly the 

 same composition as chyle, and in the higher vertebrates is gathered up 

 by the absorption by the lymphatic vessels from all parts of the body, 

 and through the skin (in some), and carried into the circulation by way 

 of the thoracic duct and the right subclavian vein. 



This lymph in the vertebrate body is made up largely from surplus 



FIG. 62. Lamprey, rock sucker, (petromyzon mar inus) much reduced. 

 Cyclostoma round mouth seven gill openings. 



nourishment not needed by the tissues. In cases where 

 some are better supplied than others with nourishment, or, 

 as in starvation, the non-vital are required to yield their 

 substance for the supply of the vital, the lymphatics do the 

 work of absorbing and re-distributing the nourishment. 



The first circulation of the mammal embryo is of colorless 

 blood and is probably the equivalent of this absorbed lymph, 

 and is absorbed largely, at any rate from the fluids of 

 the mother. 



The circulatory fluid of the lower vertebrates and higher in- 

 vertebrates, is doubtless a mixture of chyle from the di- 

 gested food and nutrient matter absorbed from without. The 

 first true differentiated lymphatic vessels occur in the cyclos- 

 tomi (lampreys, &c.). In frogs, and some other animals, large 

 pulsating lymph organs are developed just under the skin. 

 They are called lymph hearts, and they propel the lymph 

 just as the heart does the blood. 



In man, and other mammals, there are numerous glands 

 attached to the lymphatic vessels. The lymph passes through 

 these and is doubtless modified, and to a certain extent or- 

 ganized by them although they do not pulsate as do the 

 lymph hearts. The graded relationship existing between 

 man and the other animals is well illustrated and indicated 

 by these circulatory systems. 



The brain and nervous system, too, pass through the same 

 stages. 



