40 Dynamic Theory. 



develops a septum or partition in the single auricle, which, while it is 

 unfinished and imperfect, makes it the heart of a lepidosiren, and when 

 this "partition is complete the heart, with its three full cavities two 

 auricles and one ventricle is the heart of an amphibian. When finally 



FIG. 60. Lepidosiren or mud-fish, protopterus, Brazil and Africa, 18 to 20 inches long. 



FIG. 61. Ceratodus or ban-am unda. 3 to 6 feet long, of Queensland, Australia. Theso 

 two fishes belong to the order dipnoi or double breathers having both lungs and gills. 

 They have a persistent notochord and scales,, and are a cross between fishes and amphi- 

 bians. 



the ventricle is divided and the heart consists of four cavities, it is the 

 heart of a monotreme ( platypus ). Still later, when the valves are per- 

 fected so as to secure the most economical mechanical action, it is like 

 the heart of a marsupial or pouched animal. Lastly, it assumes the 

 oblique position, common to the anthropoid apes and also permanent in 

 the adult man. 



So also in the development of the urinary and sexual systems in the 

 human embryo the different stages are successively equivalent to the 

 permanent arrangements of those parts in the lampreys, the primitive 

 fishes, the dipneusta or dipnoi, the amphibia, the monotremes, the mar- 

 supials, and the edentates, and at last they become permanent in the hu- 

 man being in the same form as in the higher apes. 



Other parts of the vascular system successively reach the permanent 

 stages, as the same are fixed in the dipneusta, then the amphibia, then 

 the monotremes, &c. The gill arches are one after another suppressed 

 or changed to other use. While the last three of these arches are form- 

 ing in the embryo, the first two disappear. The last three are modified 

 into the permanent aorta and its branches, except a part that is sup- 

 pressed, and another piece of the fifth arch that becomes closed and re- 

 mains as a useless rudimentary organ called ductus venosus. 



The blood itself is a progressively developed substance. The first 

 form of blood is chyme. This, in the vertebrate animal, is simply the 

 triturated and masticated food mixed with gastric juice. In the lowest 

 animals there is probably little gastric juice in the mixture. Such as it 



