ORGANIC DEVELOPEMENT. 607 



this extended circulation will soon be wanted ; 

 and for this purpose there must be provided a 

 central organ of propulsion, or heart, the con- 

 struction of which is now commenced, at a 

 central point, by the folding inwards of a lamina 

 of the middle membrane, forming first a simple 

 groove, but, after a time, converted, by the 

 union of its outer edges, into a kind of sac, 

 which is soon extended into a longitudinal tube.* 

 The next object is to bring this tube, or rudi- 

 mental heart, into communication with the 

 neighbouring vascular trunks ; and this is effected 

 by their gradual elongation, till their cavities 

 meet, and are joined ; one set of trunks (the 

 future veins,) first uniting with the anterior end 

 of the tube ; and then another set (the future 

 arteries,) joining its other end. The addition of 

 this central tube to the vessels previously formed 

 completes the continuity of their course ; so that 

 the uniform circulation of the blood is esta- 

 blished in the direction in which it is ever after 

 to flow ; and we may now recognise this central 

 organ as the heart, which, under the name of the 

 piinctum saliens, testifies by its quick and regular 

 pulsations that it has already begun to exercise its 

 appropriate function. It is long, however, before 

 it acquires the form which it is permanently to 



* The discovery of this fact is due to Pander. See also the 

 works of Rolando, WoIfF, Prevost and Dumas, and Serres. 



