60S THE REPRODUCTIVE FUNCTIONS. 



retain ; for from being at first a mere lengthened 

 tube, presenting three dilatations, which are the 

 cavities of the future auricle, ventricle, and bulb 

 of the aorta, it assumes in process of time a 

 rounded shape, by the folding of its parts, tlie 

 whole of which are coiled, as it were, into a 

 knot ; by which means the different cavities 

 acquire relative situations more nearly corre- 

 sponding to their positions in the developed and 

 finished organ. 



The blood-vessels, in like manner, undergo a 

 series of changes quite as considerable as those 

 of the heart, and totally altering their arrange- 

 ment and distribution. Serres maintains that 

 the primitive condition of all the organs, even 

 those which are generally considered as single, 

 is that of being double, or being formed in pairs ; 

 one on the right, and another exactly similar to 

 it on the left of the middle, or mesial plane, as 

 if each were the reflected image of the other.* 



* A remarkable exemplification of this tendency to symmetric 

 duplication of organs occurs in a very extraordinary parasitic 

 animal, which usually attaches itself to the gills of the Cyprinus 

 brama, and which has been lately examined by Nordmunn, and 

 named by him the Diplozoon paradoxum, from its having the 

 semblance of two distinct animals of a lengthened shape, each 

 bent at an obtuse angle, and joined together in the form of the 

 letter X. The right and left halves of this cross are perfectly 

 similar in their organization, having each a complete and inde- 

 pendent system of vital organs ; excepting that the two alimentary 

 canals join at the centre of the cross to form a single cavity, or 

 stomach. (Annales des Sciences Naturelles, xxx, 373.) 



