ORGANIC DEVELOPMENT. 425 



wards, without any constancy.* A delicate net-work of ves- 

 sels is next formed in various parts of the area of the circle, 

 which are seen successively to join by the formation of com- 

 municating branches, and ultimately to compose larger 

 trunks, so as to establish a more general system of vascular 

 organization. But increased power for carrying on this ex- 

 tended circulation will soon be wanted; and for this purpose 

 there must be provided a central organ of propulsion, or 

 heart, the construction of which is now commenced, at a 

 central point, Dy the folding inwards of a lamina of the mid- 

 dle 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.t 

 The next object is to bring this tube, or rudimental heart, 

 into communication with the neighbouring vascular trunks, 

 and this is effected by their gradual elongation, till their ca- 

 vities 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 established in the direc- 

 tion in which it is ever after to flow; and we may now re- 

 cognise this central organ as the heart, which, under the 

 name of the punctum 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 tlie form which it is permanently to retain; for 

 from being at first a mere lengthened tube, presenting three 

 dilatations, which are the cavities of the future auricle, ven- 

 tricle, and bulb of the aorta, it assumes in process of time a 

 rounded shape, by the folding of its parts, the whole of 



* These phenomena are similar to those which were noticed as presented 

 by the larvae of some insects and other inferior animals. 



f The discovery of this fact is due to Pander. See also the works of Ro- 

 lando, Wolff, Prevost and Dumas, and Serres. 



V OL. II. 54 



