THE VASCULAR SYSTEM. 633 



present in a well-developed condition. As pointed out by the 

 Hertwigs, a vascular system is always absent where there is not 

 a considerable development of connective tissue. 



As to the ontogeny of the vascular channels there is still much to be 

 made out both in Vertebrates and Invertebrates. 



The smaller channels often rise by a canalization of cells. This process 

 has been satisfactorily studied by Lankester in the Leech 1 , and may easily 

 be observed in the blastoderm of the Chick or in the epiploon of a newly 

 born Rabbit (Schiifer, Ranvier). In either case the vessels arise from a net- 

 work of cells, the superficial protoplasm and part of the nuclei giving rise 

 to the walls, and the blood-corpuscles being derived either from nucleated 

 masses set free within the vessels (the Chick) or from blood-corpuscles 

 directly differentiated in the axes of the cells (Mammals). 



Larger vessels would seem to be formed from solid cords of cells, the 

 central cells becoming converted into the corpuscles, and the peripheral cells 

 constituting the walls. This mode of formation has been observed by 

 myself in the case of the Spider's heart, and by other observers in other 

 Invertebrata. In the Vertebrata a more or less similar mode of formation 

 appears to hold good for the larger vessels, but further investigations are 

 still required on this subject. Gotte finds that in the Frog the larger vessels 

 are formed as longitudinal spaces, and that the walls are derived from the 

 indifferent cells bounding these spaces, which become flattened and united 

 into a continuous layer. 



The early formation of vessels in the Vertebrata takes place in the 

 splanchnic mesoblast; but this appears due to the fact that the circulation 

 is at first mainly confined to the vitelline region, which is covered by 

 splanchnic mesoblast. 



The Heart. 



The heart is essentially formed as a tubular cavity in the 

 splanchnic mesoblast, on the ventral side of the throat, immedi- 

 ately behind the region of the visceral clefts. The walls of this 

 cavity are formed of two layers, an outer thicker layer, which has 

 at first only the form of a half tube, being incomplete on its 

 dorsal side; and an inner lamina formed of delicate flattened 

 cells. The latter is the epithelioid lining of the heart, and the 

 cavity it contains the true cavity of the heart. The outer layer 

 gives rise to the muscular wall and peritoneal covering of the 

 heart. Though at first it has only the form of a half tube (fig. 



1 "Connective and vasifactive tissues of the Leech." Quart. J. of Micr. Science, 

 Vol. xx. 1880. 



