226 PRINCIPLES OF ANIMAL BIOLOGY 



the corresponding positions in later stages is derived from these layers. 

 Above the endodenn, between it and the notochord, two layers of the 

 mesoderm approach one another and form the mesentery (Fig. 163) which 

 later suspends the digestive tract in a sack of peritoneum. In the longi- 

 tudinal section, Fig. 181, the mesoderm is not represented above the 

 digestive tract, since the section passes exactly through the median plane. 

 But below the intestine the mesoderm occurs, divided into its two char- 

 acteristic layers. 



Digestive Tract and Its Derivatives.^From the invaginated vegeta- 

 tive side of the embryo, which forms the endodcrm, is formed most of 

 the lining of the digestive tract, and the lining of organs which branch 

 off from the digestive tract, such as the gill pouches, the liver, lungs, 

 pancreas, and some others. 



Gill Pouches. — The gill pouches, represented as seen from above in 

 Fig. 182, are evaginations from the sides of the pharynx, or anterior part 

 of the gut. Tj'^pically there are five of these protrusions on each side, but 

 some of them are often rudimentary, or two of them may be nearly com- 

 bined, so that the number frequently appears to be less. Successive 

 stages in the evagination of the gill pouches are shown in Fig. 182, A, B, 

 C. They finally reach the ectoderm, with which they fuse. In fishes 

 and usually in amphibians the ectoderm and endoderm both break open 

 at the point of fusion, so that the pharynx is open to the outside. These 

 openings are the gill clefts. They serve as channels for the passage of 

 water, which enters at the mouth (not shown in the figiu'e since it is at 

 a lower level). The course of the water is indicated in the figure by ar- 

 rows. In the fishes and in at least the young stages of amphibians gills 

 are developed upon the tissue (gill bars) between the gill clefts. The 

 gills are organs for the absorption of oxygen. In the production of cer- 

 tain types of gill the endoderm of the gill pouches has a share. 



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- 

 functional organs. Thus, the first pouch becomes, as is pointed out 

 below, part of the Eustachian tube and middle ear. Others share in the 

 production of the tonsils, the thymus, and the parathj^roid glands. 



Mouth. — The mouth starts as an invagination of the ectoderm from 

 the outside, as in Fig. 183, m. For a time it is separated from the rest 

 of the digestive system by a memln-ane composed of an outer layer 

 of ectoderm and an inner layer of endoderm. This mem])rane later 

 breaks (I'ig. 184), 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. 



