7 6 



animal now existing. This is especially evident when we compare 

 particular organs. Thus, the heart is at first a simple sac, formed 

 from a mass of cells by the breaking down of the interior ones, and 

 so forming a tube, the permanent condition of the heart in the 

 higher radiata, and in the lower articulata and mollusca; as develop- 

 ment proceeds, the one cell becomes divided into two, an auricle 

 and ventricle ; the stage is then reached which occurs in passing 

 from the tunicata to the higher mollusca. The next stage seems 

 to me the most remarkable of all, and totally inexplicable on any 

 other theory than that of descent with gradual modification and 

 improvement. The embryo has then reached the stage of the fish. 

 Now fishes are aquatic animals, and their respiratory organs are 

 totally different from the organs of those that breath air. Gills 

 or branchiae occupy slits in the neck, over which the water flows, 

 so that the blood may obtain the oxygen it requires. Man is an air 

 breathing animal, yet in the human embryo, at this stage of devel- 

 opment, slits appear in the neck, with the branchial arches of rudi- 

 mentary gills. As the process goes on, some of these arched blood- 

 vessels disappear, others are changed into the vessels that supply 

 the rudimentary lungs, the head, and the arms. 



The lungs of air-inhaling animals are homologous with the swim 

 bladder of fishes. In the sauroid family of fishes, now represented 

 by the Lepidosteous and Polypterus, some of the branchial arches 

 send prolongations into the swim bladder. As we rise from the 

 amphibious to the higher reptiles, the lungs become more de- 

 veloped, and we have a heart with three chambers. Through 

 this stage, also, the mammalian embryo passes ; the four chambered 

 heart of a mammal being only fully completed at the moment when 

 the valve of the foramen ovale closes with the first aerial inspiration. 

 What is true of the heart is also true of the other great organs. — 

 The nervous tissues, the brain, and spinal cord, follow the same 

 law. The comparative anatomist can point out the gradual modi- 

 fication of these organs through a similar series of changes. The 

 brain of the fish is so like to that of the human embryo in its early 

 stages, that the one is indistinguishable from the other. Its changes 

 of form represent successively the permanent forms in the reptile, 



