198 PROFESSOR MARSHALL AND EDWARD J. BLES. 



diverticulum which is in close relation with the epiblast of the 

 stomatoda^al invagination. 



There are five gill pouches on each side of the pharynx. These 

 pouches are double folds of hypoblast, forming vertical partitions 

 projecting outwards from the pharynx to the surface epiblast. The 

 cavity of the pharynx extends a very short distance outwards 

 between the two layers of each fold, and along almost their entire 

 extent the two layers are in close contact with each other. The 

 outer ends of the pouches reach the epiblast and fuse with its inner 

 or nervous la} r er.' 



As seen in horizontal section, the gill pouches radiate outwards 

 from the centre of the pharynx. The most anterior pair are the 

 hyomandibular pouches or clefts ; and then in succession come the 

 first, second, third, and fourth branchial clefts, the hindmost or 

 ourth branchial cleft being always small, and in some specimens 

 indistinct. The pharynx is widest opposite the first branchial arch, 

 behind which it narrows rapidly. Behind, or rather between the 

 branchial arches of the fourth pair, the pharynx passes back into the 

 oesophagus, a narrow tube from the ventral border of which the lungs 

 are already commencing to arise as lateral diverticula. 



The anterior part of the body, immediately behind the constricted 

 neck, presents a rounded swelling on each side, caused by the head 

 kidney. 



3. The Heart and Pericardial Cavity. 



The pericardial cavity lies in the floor of the pharynx, in front of 

 the commencing liver, and opposite the four pairs of branchial arches. 

 Its side walls are very thin, as also is its floor except where it 

 is thickened in front by the sucker (Fig. 4, S). In sagittal 

 section it is conical in shape, with the apex anterior. It is in 

 communication behind with the general body cavity, which at this 

 stage consists, owing to the great bulk of the food-yolk, of very 

 narrow chink-like spaces on either side of the body. 



The pericardial cavity is lined by a single layer of flattened 

 epithelial cells. Its dorsal part is divided into right and left halves 

 by a median mesocardial septum, the mode of development of which 

 has been explained in the preceding section. 



The heart (Plate XIV., Fig. 4) is a tube slightly twisted on itself, 



