426 THE REPRODUCTIVE FUNCTIONS. 



which are coiled, as it were, into a knot; hy which means 

 the different cavities acquire relative situations more near- 

 ly corresponding 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 to- 

 tally altering their arrangement and distribution. Serres 

 maintains that the primitive condition of all the organs, even 

 those which are generally considered as single, is that of be- 

 ing 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.* Such is obviously the permanent condition of all 

 the organs of. sensation, and also of the apparatus for locomo- 

 tion: and it has just been shown that those portions of the 

 nervous system which are situated in the mesial plane, such 

 as the spinal cord and the brain, consisted originally of two 

 separate sets of parts, which are brought together, and con- 

 joined into single organs. In like manner we have seen that 

 the constituent laminae of the heart are at first double, and 

 afterwards form, by their union, a single cavity. The ope- 

 ration of the same law has been traced in the formation of 

 those vascular trunks, situated in the mesial plane, which are 

 usually observed to be single, such as the aorta and the vena 

 cava: for each were originally formed by the coalescence of 

 double vascular trunks running parallel to each other, and 

 at first separated by a considerable interval; then approach- 

 ing each other, adhering together, and quickly converted, 



* A remarkable exemplification of this tendency to symmetric duplication 

 of organs occurs in a very extraordinary parasitic animal, which usually at- 

 taches itself to the gills of the Cyprinus Irama, and which has been lately 

 examined by Nordmann, 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 independent 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.) 



