DEVELOPMENT OF CIRCULATING APPARATUS. 



589 



walls of both, the materials requisite for its growth j but there 

 is no direct communication between the two. The same means 

 serve for the aeration of the blood of the embryo ; for this, 

 being brought from its body in the venous condition, is exposed 

 to the influence of the arterial blood of the parent, through 

 the thin walls of its vessels, just as the venous blood of aquatic 

 animals is aerated in their gill-tufts, and passes back to the 

 embryo in the arterial condition, having imparted its carbonic 

 acid to the blood of the parent, 

 and received from it oxygen. 

 Thus all but the very early stages 

 of development are performed in 

 Mammals, by means of which we 

 scarcely find a trace in Oviparous 

 animals ; yet the ova of both are 

 originally formed on the same plan, 

 and the first changes which they 

 undergo are exactly analogous. 



762. It would not be consistent 

 either with the design or with the 

 limits of this work, to enter in 

 much detail into the considera- 

 tion of the processes of develop- 

 ment, although they present many 

 points of the highest interest. 

 The general history of the evolu- 

 tion of the Circulating apparatus 

 and of the Nervous centres may, 

 however, be noticed, as character- 

 istic examples of the mode in 

 which the evolution of the several 

 organs of the body takes place. 

 The Heart, in Man and other 

 Mammals, as in the Bird ( 758), 

 is at first a simple tube, resembling 

 the pulsatile trunk that remains as 



the sole organ of impulsion in the lowest forms of circulating 

 apparatus. After a time this tube is doubled upon itself, and 

 two cavities are formed, an auricle and a ventricle ; in this con- 

 dition, it strongly resembles the heart of the Fish ( 286). The 

 circulation too is, at an early period, that of the Fish ; for the 



Fig. 326. EMBRYO OP THE FOWL, 

 from the Ovum shown in fig. 

 318, greatly enlarged : 



a, b, folds of irerminal membrane, 

 enveloping head and tail; c, la- 

 teral folds; d, e, rudiments of 

 optic ganglia and cerebrum ; f, 

 heart ; g, dilated termination of 

 venous trunk, forming atrium of 

 heart; A, aorta; 1, 2, 3, 4, bran- 

 chial arches; i, i, vessels of 

 vascular area; k, k, dorsal lami- 

 nse; I, I, rudiments of vertebral 

 arches. 



