ORGAN DEVELOPMENT IN VERTEBRATES 257 



process of gastrulation (but see p. 261). At first the endoderm forms only 

 a trough, which is covered dorsally by the mesodermal layer of which 

 the notochord is a part, but fairly soon the edges of the trough grow 

 round to meet under the notochord, so as to form a closed tube made 

 wholly of endoderm. The anterior part of this tube reaches forward under 

 some of the head structures which have just been described. At its tip, 

 the tube is at first closed, and in this region there is no mesoderm lying 

 between it and the ectoderm. In the mesoderm-free area, the gut first 

 fuses with the ectoderm, and then both layers break down, so that an 

 opening appears leading from the exterior into the lumen of the gut; 

 this is the rudiment of the mouth. 



Just posterior to this, the gut becomes swollen into a large cavity, which 

 is known as the pharynx. The wall of this becomes thrown into a series of 

 deep folds, which run from top to bottom on each side. These folds, five in 

 number, eventually reach through the mesoderm to the ectoderm, and 

 fuse with it. Again, the combined ectoderm and endoderm breaks down 

 and forms an opening. In this case, the openings correspond to the gill 

 openings offish; in the higher groups of vertebrates they make only a 

 transitory appearance during early embryonic life, before becoming 

 transformed into something else; in some cases they never open com- 

 pletely at any stage. Their fate in the various groups is too complex to 

 be followed in detail here. (For instance, the most anterior forms part 

 of the Eustachian tube and the tympanic cavity of the middle ear.) 

 Between each of the gill slits (which are also known as pharyngeal or 

 visceral clefts) is a gill 'arch'. In these there is a core of mesoderm between 

 the ectoderm and endoderm, and in the centre of this core, an important 

 blood vessel, one of the so-called aortic arches, will eventually run. 



Essentially the same structures are formed in the chick, but by somewhat 

 different processes, since the anterior part of the gut, as we have seen 

 (p. 252) appears not during gastrulation but in connection with the head 

 fold. The formation of the mouth and pharynx, however, involves the 

 same processes of the local fusion of ectoderm and endoderm, followed 

 by their breaking down to give place to an opening. 



In the cliick, another derivative of the pharynx makes its appearance 

 at a fairly early stage. A pocket of endoderm pushes out from the floor 

 towards the posterior end of the swollen pharyngeal region, and rapidly 

 elongates and extends backwards; it soon branches into two. This is the 

 rudiment of the trachea leading to the two lungs. In the Amphibia it 

 arises in the same region and in a similar way, but at a considerably later 

 stage. 



