312 Dr Allen Thomson o/t the Vascular System 



The contractions of tlie heart become gradually more rapid, 

 amounting to 50 or 55, at the time when the animal is about four 

 weeks old, and the external gills are nearly perfect. The colour 

 of the blood deepens. The contraction of the heart is now no 

 longer uniform from one end to another ; for as the auricle con- 

 tracts and discharges its contents, the ventricle is dilated, and 

 when this cavity contracts, the auricle in its turn expands for 

 the reception of a new supply of blood, which flows into it from 

 the venae cavae. By comparing the slight sketches which are 

 given in Plate III. of the heart of the Newt, at three sepa- 

 rate periods, — before it comes out of the egg, — when it has just 

 left it, — and when the external gills are nearly perfect, with those 

 at the corresponding periods in the Frog, the great resemblance 

 they bear to one another cannot fail to be apparent. 



It is to be regretted that very few observations have been 

 made on the development of the Ova of the Higher Orders of 

 Reptiles, as, from the great variety in their forms, we might have 

 hoped that the investigation of their evolution would have thrown 

 considerable light on the mode in which this process is effected 

 in other animals. According to the observations of Emmert and 

 Hochstetter *, of Rathke f , and of Baer %, it has been shewn that 

 in the Lizard tribe the surface of the yolk is covered with the 

 ramifications of the omphalo-mesenteric arteries at a very early 

 period. It has also been ascertained, that in these animals the 

 heart, which in the adult generally consists of two auricles and 

 a ventricle, divided by a septum into two compartments, has at 

 first only a single auricle and ventricle ; and that these cavities 

 become divided by the gradual formation of partition walls be- 

 tween them. In the early periods there is only a single arte- 

 rial vessel arising from tlie ventricle, the bulb of the aorta, 

 which afterwards becomes divided so as to form the roots of the 

 right and left aortae with the other arterial vessels §. 



* Untersuchungen uber die Entwickelung der Eidechsen, &c. Ileil's Ar- 

 chiv. X. p. 84. 



-j- In his Memoir on the Development of the Respiratory Organs in the 

 Isis, in the Repert. Gener. d'Anat. torn, vii., and Edin. Med. and Surg. Joiirn. 

 January 1830. 



$ Loc. cit. 



§ The division of these vessels will form one of the subjects treated of in 

 the second part of this paper. 



