210 



PRINCIPLES OF ANIMAL BIOLOGY 



is indicated in the figure by arrows. In the fishes and in at least the 

 young stages of amphibians, gills (organs of respiration) are developed 

 upon the tissue {gill bars) between the gill clefts. 



In the higher vertebrates the gill pouches do not open to the outside 

 at all or do so only temporarily. They are to be regarded as to some 

 extent vestigial organs, an inheritance of an ancestral condition in which 

 functional gills were present. However, some of them are regularly 

 converted during embryonic development into other functional or non- 



an 



Fig. 183. — Longitudinal section of the early embryo of a frog, diagrammatic, an, anus; 

 br, brain; c, coelom; ec, ectoderm; en, endoderm; int, intestine; li, liver; ms, mesoderm; 

 nd, notochord; sp, spinal cord; st, stomach. 



functional organs. Thus the first pouch becomes part of the Eustachian 

 tube and middle ear. Certain of the bars share in the production of the 

 tonsils, the thymus, and the parathyroid glands. 



The mouth starts as an invagination of the ectoderm from the outside, 

 as in Fig. 185m. For a time it is separated from the rest of the digestive 

 system by a membrane composed of an outer layer of ectoderm and 

 an inner layer of endoderm. This membrane later breaks, and part 

 of the fore end of the gut is incorporated in the mouth cavity. That 

 part of the mouth derived from the external invagination is of course 

 lined with ectoderm. 



Outgrowths of the Digestive Tract. — The liver appears at an early 

 stage as an evagination from the lower side of the intestine just behind the 

 stomach. In the frog the liver is present shortly after the fusion of the 

 neural folds (see Fig. 183, li). An early indication of the liver is also 

 shown in Fig. 185, li. This pouch grows in extent and soon becomes 

 branched. One branch at the posterior side of the liver forms the gall 

 bladder (gb). The rest are bound together by mesodermal tissue which 

 collects about them, forming part of the body of the liver. The undivided 



