520 THE HEART. 



The blood systems may either be closed or communicate with 

 the body cavity. In cases where the primitive body cavity is atro- 

 phied or partially broken up into separate compartments (Insecta, 

 Mollusca, Discophora, etc.) a free communication between the vas- 

 cular system and the body cavity is usually present ; but in these 

 cases the communication is no doubt secondary. On the whole it 

 would seem probable that the vascular system has in most instances 

 arisen independently of the body cavity, at least in types where the 

 body cavity is 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 

 1 oru Rabbit (Schafer, Ranvier). In either case the vessels arise from a net- 

 work of cells, the superficial protoplasm and part of the miclei 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 a.xes of the cells (Mammals). 



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

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

 constituting the walls. Tliis 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. Gb'tte 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 cii'cula- 

 tion 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, immediately 

 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 



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

 Vol. xx. 1880. 



