216 PROFESSOR MARSHALL AND EDWARD J. BLES. 



from which they have recently emerged ; and sections of the sucker 

 show that the greatly elongated and columnar cells of the sensory 

 layer of the epiblast covering them are often produced at their free 

 ends into protoplasmic processes, that would seem well fitted for 

 absorbing the jelly. We have also found the pharynx at this and 

 at earlier stages to be very commonly completely filled, in hardened 

 specimens, with a dense and coherent coagulum, the material for 

 which may be obtained in the way suggested above. 



2. Internal Anatomy. 



The pharynx is still of large size, being wide both laterally and 

 dorso-ventrally (Figs. 8, 9, 10, P). It is, however, relatively to 

 the whole head, smaller than at 5 mm., owing to the increased size 

 of the brain. In front of the gill clefts the pharynx is compressed 

 laterally, and prolonged forwards beneath the brain to the septum 

 dividing it from the stomatodeeal depression. At this septum (Fig. 

 10, S T) the epiblast and hypoblast are in close contact with each 

 other, the hypoblast and the deeper or nervous layer of the epiblast 

 fusing indistinguishably with each other. 



At its hinder end, opposite the ears, the pharynx narrows rapidly 

 and passes into the oesophagus, which has a very small lumen, 

 appearing in transverse sections as a vertical slit. 



The gill slits (Fig. 9) are much as in the previous stage, the two 

 layers of hypoblast bounding each slit being still in contact along 

 almost their whole width. Along the outer edges of the gill slits 

 the hypoblast cells fuse completely with the deeper or nervous layer 

 of the surface epiblast. Well marked but shallow grooves of the 

 epiblast mark the position of the gill clefts on the surface. The 

 hyomandibular cleft (Fig. 9, C H) is very similar to the hinder or 

 branchial clefts, but is connected with the surface epiblast for a 

 shorter distance than these. 



The thyroid body is present as a well-marked median diverticulum 

 of the ventral wall of the pharynx, near its anterior end. It is 

 directed backwards and very slightly downwards, and does not 

 quite reach the front wall of the pericardial cavity. 



The lungs are more prominent than before, as lateral outgrowths 

 from the ventral wall of the oesophagus in the neck region. The 

 inner surface of their lining epithelium is strongly pigmented. 



