The Comparative Physiology of tlie Pituitary Body 275 



the tubules are found to consist of columns of cells surrounding blood spaces. 

 The lumen is a blood channel. This feature has been emphasised by 

 Gentes, who states that the elasmobranch pituitary is a typical example of 

 a gland whose secretion is poured directly into the blood-vessels. The 

 epithelial cells surrounding the blood channels are columnar, with nuclei 

 situated at their bases, the part of the cell bordering on the blood-vessel 

 being clear. Outside the columnar cells and separating the vascular tubules 

 from one another is a small amount of what appears to be a very vascular 

 connective tissue. The vascular tubules and this connective tissue make 

 up the body of the lobe. There are no deeply staining granular cells 

 resembling the chromophil cells of the anterior lobe of the pituitary of 

 mammals and teleosts, nor are there any cells exactly resembling those 

 of the pars intermedia. 



The anterior extremity of the skate's pituitary consists of a comparatively 

 thin prolongation of epithelium enclosing a cavity (tig. 3, b, of Plate) which 

 is stated by Gentes to be the remains of the cavity of the original sac from 

 which the pituitary develops. It is lined by columnar epithelium very 

 similar in appearance to that surrounding the blood-vessels in the body 

 of the lobe. The cavity appears to be completely closed, and is much 

 sacculated by convolutions of its wall. Outside this sac are numerous 

 blood-vessels. 



There is no differentiation of the pituitary gland of the skate into 

 anterior and posterior lobes. An infundibular cavity is present which 

 runs backwards and downwards to the body of the pituitary. It does 

 not penetrate into the pituitary, but ends in the middle line, as shown in 

 the figure. When followed laterally, however, the infundibular cavity is 

 found to pass on either side into the saccus vasculosus. The nearest 

 approach to anything resembling a posterior lobe is seen in the thin lamina 

 of nervous tissue which bounds the infundibular cavity and passes into the 

 tissue of the pituitary. Whether the line vascular tissue that lies between 

 the epithelial tubules in the body of the pituitary is of nervous origin or 

 not is uncertain, but it is continuous with the lamina of nerve tissue that 

 forms the lower wall of the infundibular cavity. If this tissue really be- 

 longs to the posterior lobe, then we have in the skate a very complete inter- 

 mixture of elements derived from the brain and from buccal epithelium. 

 Gentes states that the posterior lobe is completely absent in elasmobranchs. 

 It is probable, however, that some representative of the processus in- 

 fundibuli exists even in adult life, and that the general plan of development 

 of the pituitary in elasmobranchs does not differ from that of other verte- 

 brates. The nerve tissue in the anterior wall of the infundibulum must be 

 regarded as a representative, in part at least, of the posterior lobe. But 

 its constitution is altered by the large developnient of the saccus vas- 

 culosus, and the extension of the epithelium of the saccus over the lining 

 wall of the infundibulum. 



The saccus vasculosus is extivmrly well developed in the skate, and is a 



VOL. I., \(). :5. — 1908. l'> 



