THE VASCULAR SYSTEM. 355 



we have already frequently indicated, as to the origin of 

 the four secondary germ-layers ; it clearly shows that the 

 skin-fibrous layer originates from the exoderm, the intes- 

 tinal-fibrous layer, on the contrary, in an analogous manner, 

 from the entoderm of the gastrula ; the cavity thus caused 

 between the two fibrous layers is the first rudiment of the 

 body-cavity, or the coelom (Figs. 50, 51, vol. i. p. 236). As the 

 Amphioxus larva thus shows that the fission of the layers 

 is the same in the lowest Vertebrates as in the Worms, it at 

 the same time represents the phylogenetic connection be- 

 tween the Worms and the higher Vertebrates. As, more- 

 over, the primitive vascular stems in the Amphioxus 

 originate in the intestinal wall, and in this, as in the em- 

 bryos of all other Vertebrates, proceed from the intestinal- 

 fibrous layer, proof is afforded us that the earlier embryolo- 

 gists were right in calling the latter the vascular layer. 

 Finally, the Comparative Ontogeny of the different verte- 

 brate classes further convinces us that the vascular layer 

 is originally everywhere the same. The vascular system in 

 Man, as in all Skulled Animals, forms a complex apparatus 

 of cavities, which are filled with juices, or fluids, containing 

 cells. The vessels play an important part in the nourish- 

 ment of the body ; some of them conduct the nutritive 

 blood fluid round in the different parts of the body (blood- 

 vessels) ; some collect the wasted juices and discharge them 

 from the tissues (lymph-vessels). With the latter, the 

 great " serous cavities " are also connected, especially the 

 body-cavity, or cceloma. The heart, acting as a centre of 

 motion for the regular circulation of the juices, is a strong 

 muscular pouch, which contracts in regular pulsations, and 

 is provided with valves, like those of a pump apparatus. 



