THE VASCULAR SYSTEM. 99 



the nervous centres. The remaining portions of the masses are converted 

 into muscular tissue and integuments. The spinal chord also, as well as 

 the chorda dorsalis, is provided with a kind of sheath, but not a fibrous 

 one, such as is possessed by the latter structure. 



Development of the Vascular System.* 



Formation of the Heart. The account given by Reichert \ of the mode 

 of development of the heart in frogs, has been, for the most part, con- 

 firmed by the investigations of Vogt J on the development of the toad, 

 and those of Kolliker on the development of the Batrachians generally. 

 An almost similar mode of development is observed also in fishes, as 

 shewn by the researches of Vogt on the development of the young 

 salmon. || According to each of these observers, the heart, in its earliest 

 formation, is composed of a solid compact mass of embryonic cells, similar 

 to those of which the other organs of the body are constituted. It is at 

 first unprovided with a cavity : but this shortly makes its appearance, re- 

 sulting apparently from the separation from each other of the cells of the 

 central portion. A liquid is now formed in the still closed cavity, and the 

 central cells may be seen floating within it. These contents of the cavity 

 are soon observed to be propelled to and fro with a tolerable degree of 

 regularity, owing to the commencing pulsations of the heart. 



These pulsations, according to Vogt, take place even before the ap- 

 pearance of a cavity, and immediately after the first " laying down" of 

 the cells from which the heart is formed. Vogt has observed this espe- 

 cially in the embryonic salmon. H At first the contractions seldom exceed 

 from fifteen to eighteen in the minute. In the production of them, the 

 whole mass of cells appears to be concerned, for in none of the individual 

 cells is there ever observed either contraction or enlargement during the 

 pulsations : although M. Dumesny** states, as the result of observations on 

 the embryos of Psecilia Surinami, that such contractions and dilatations of 

 the several cells may be distinctly observed. The occurrence of con- 

 tractions of the walls of the heart previous to the formation of any muscu- 

 lar or other contractile tissue has been observed also in the chick by MM. 

 Prevost and Lebert.jf 



Probably by the metamorphosis of the contained cells, the fluid within the 

 cavity of the heart shortly assumes the characters of blood. At the same 



* Mullet's Physiology, p. 1620. t Mullet's Physiology, p. 1526. 



Entwickelungs-geschichte der Geburtshelferkrote, p. 69. 



An. desSc. Nat. 1846, Zoologie, p. 96. 



II Hist. Nat. des Poiss. d'eau douce, par M. Agassiz, tome i. 1842, p. 181. 



f[ Op. cit. p. 182. ** An. des Sc. Nat. 1844, p. 344. 



ft Ibid. 1844, p. 302. A long account of the mode of development of the heart, and of 

 the subsequent changes in its form, and in the relations of its several parts is contained in 

 this memoir. 



H 2 



