Comparative Development. 39 



The blood passes into -the rear or lower cavity of the heart by two prin- 

 cipal veins leading from the yelk sac, and passes out from the front 

 cavity by two aortae.that subdivide into five gill blood vessels, which, 

 passing up by way of the gill arches, unite above and pass backward 

 above the intestine. In the fishes this arrangement is permanent and is 



FIG. 59. Adult ascidian. a. Anus. cm. -Eye. c. Gill 

 cavity c/. Cloacal cavity. d.~ Intestinal tube. e. -Ovary. 

 M.~ lestes. hz. Heart. z.-Eggs. &.-Gills. Z&.-Liver. 

 m. Medullary tube, mg Stomach, tnt- -Mantle, o. 

 Mouth, q Cloacal opening, si. Seed duct. ,sm. Open- 

 ing of seed duct, t Aorta (dorsal bloodvessel)- u Side 

 canals, v Intestinal vein (ventral blood vessel), iv. 

 Koot-hke attachments of the mantle, z. Boundary 

 between gill intestine and stomach intestine, y. 

 Hypo branchial groove, z. Embryos of the ascidian. 

 necessary to convey the blood to the gills, to 

 be oxydized by contact with the water. But 

 as it has no such use to the human being, 

 or other mammals, nor to birds, nor to rep- 

 tiles, except some amphibians, and reptile 

 larvae, such as tadpoles; a part of these gill 

 blood vessels and the gill arches themselves, 

 are entirely suppressed or else turned to other 

 use. A part of the vessels persist and are 

 found in all vertebrates, including man see 

 figs. 26, 27, 28, 29. The gill leaves are not 

 developed in the embryo of any amnion 

 animal. 



In connection with this modification of the 

 fish breathing apparatus in the human embryo 

 is the development of the lungs. These oc- 

 cupy the same place and are formed in the 

 same way as the swim bladder of fishes, from 

 the ventral side of the forward end of the in- 

 testine just back of the gills. At first a single 

 vesicle, it soon divides into two, one left and 

 one right. All vertebrates have this vesicle 

 except the amphioxus and the cyclostomi, 

 the lowest two orders. 



In the adult fishes this vesicle remains permanently filled with air, 

 the amount of which may be increased or diminished by the action of 

 the animal. In many fishes the bladder is double. In the mud-fishes, 

 dipneusta, this swimming bladder first becomes true lungs. In the cer- 

 atodus, the Australian mud-fish, a low form (but large, being six feet 

 long), the lung is single. But in the African mud-fish, protopterus, 

 and the American mud-fish, lepidosiren, the lungs are double, and so 

 they are in all the higher vertebrates, including man. 



The heart of the human embryo, after passing through the fish stage, 



