426 THE REPRODUCTIVE FUNCTIONS. 



which arc coiled, as it were, into a knot; by wliich means 

 the difierent cavities acquire relative situations more near- 

 ly corresponding to their positions in the developed and 

 finislicd orjran. 



Tlie 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 

 7nesial 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 arc brought together, and con- 

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

 the constituent laminse 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 wer^ originally formed by the coalescence of 

 double vascular trunks running parallel to each other, and 

 at first separated hy a considerabl-e interval; then approach- 

 ing each other, adhering together, and quickly converted, 



• A remarkable exemplification of this tendency to symmetric duplication 

 oforg-ans occurs in a very extraordinary parasitic animal, which usually at- 

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

 examined by Nordmann, and named by him tl>e Dipluzoon paradoxum, from 

 its having- the semblance of two distinct animals of u lengthened shape, each 

 bent at an obtuse angle, aiul joined together in the form of the letter X. The 

 right and left halves of this cross are perfectly similar in their organization, 

 leaving 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 stomacli, (Aimales des Sciences NatureHes, xxx. o7o.) 



