»540 THE REPRODUCTIVE FUNCTIONS. 



nearly corresponding to their positions in the de- 

 veloped and finished organ. 



The blood-vessels, in like manner, nndergo a 

 series of changes quite as considerable as those of 

 the heart, and totally altering their arrangement 

 and distribution. Serres maintains that the pri- 

 mitive 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 re- 

 flected image of the other.* Such is obviously the 

 permanent condition of all the organs of sensation, 

 and also of the apparatus for locomotion ; 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, con- 

 sisted originally of two separate sets of parts, which 

 are brought together and conjoined 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 operation of the same law has been traced in 

 the formation of those vascular trunks, situated in 



* 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 hrama, and 

 which has been lately examined by Nordmann, and named by him 

 the Diplozoon paradoxian, 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 organiza- 

 tion, 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.) 



