168 ESSENTIALS OF ZOOLOGY 



supported on special mesenteric folds of the peritoneum. 

 These have been called, in the male, the mesorchium; in the 

 female, the mesoarium. 



Mesenchyme is yielded from the splanchnocoel mesoderm 

 generally, and the musculature of the visceral arches, as of the 

 viscera, is likewise derived from the body-cavity mesoderm. 

 The mesenchyme is otherwise resolved into connective tissues 

 and into the blood and the vascular system. 



The vascular system is derived, as has just been said, from 

 mesenchyme. Certain cells are freed as corpuscles, while 

 others are converted into the blood-vessels. The heart and 

 the main vessels are furnished with coats of connective tissue 

 and of muscle. The system is arranged in a heart and a wide- 

 spread series of arteries leading blood from the heart, capil- 

 laries which bring the blood into intimate relationship with the 

 cells of tissues and organs, and veins which return the blood 

 to the heart. Some of the veins are widely expanded, and 

 such are called sinuses. The blood is red-coloured, haemo- 

 globin being contained in the oval, flat, nucleated, red cor- 

 puscles, or erythrocytes. The other blood cells are leucocytes 

 and thrombocytes. The fluid in which they occur, the plasma, 

 together with leucocytes, exudes through the capillary walls 

 into the spaces outside them, bearing oxygen and food which 

 thus come into close contact with the cells. The escaped 

 fluid cannot return against the current, and, now called lymph, 

 it is collected in spaces of the mesenchyme. These spaces 

 communicate with better-defined spaces, and the latter with 

 special vessels called lymphatics. The lymphatics restore the 

 lymph to the blood by an opening on each side into the dorsal 

 wall of the precaval sinus, an opening guarded by two valves. 

 The valves open when the pressure of the lymph is sufficient 

 to overcome the pressure of the blood which keeps the valves 

 closed. 



The circulation primarily is concerned in bringing the food 

 material from the yolk into the embryo. The first veins are 

 a pair of vitelline veins which lead from the blastoderm to the 

 embryo and merge to form the heart. In front two vessels 

 are formed which diverge from the heart to encircle the head 

 fold of the gut and turn backwards to supply the embryo. The 



