EXPLANATION OF PLATE 19. 477 



bodies the suprarenal bodies of the higher Vertebrata are homo- 

 logous. This question I shall not attempt to answer in a definite 

 way. My own decided belief is that the suprarenal bodies of 

 Scyllium are homologous with the suprarenal bodies of Mammalia, 

 and a good many points both in their structure and position 

 might be urged in favour of this view. In the mean time, how- 

 ever, it appears to me better to wait before expressing a definite 

 opinion till the embryonic development of the suprarenal bodies 

 has been worked out in the higher Vertebrata. 



EXPLANATION OF PLATE 19. 



COMPLETE LIST OF REFERENCE LETTERS. 



Nervous System. 

 //. Nerve, spn. Spinal nerve, sy g. Sympathetic ganglion. 



Alimentary Canal. 



cl. Cloaca. in d. Cloacal involution. as ep. OZsophageal epithelium, pan. 

 Pancreas. ///. Thyroid body. 



General. 



al> p. Abdominal pocket (pore), am: Auricle. cav. Cardinal vein. can v. 

 Caudal vein. ly. Lymphoid tissue, mm. Muscles, od. Oviduct. /(".Pericardium. 

 pp. Body cavity, s >: Suprarenal body. //. Ureter, v ao. Ventral aorta (anterior 

 continuation of bulbus arteriosus). ven. Ventricle, wd. Wolffian duct. 



Figs, la, ii>, if. Three sections through the cloacal region of an embryo belong- 

 ing to stage O. i a is the anterior of the three sections. Zeiss A, ocul. 2. Reduced 

 one-third. 



i a shews the cloacal involution at its deepest part abutting on the cloacal section 

 of the alimentary tract. 



i /MS a section through a point somewhat behind this close to the opening of the 

 Wolffian ducts into the cloaca. 



r c shews the opening to the exterior in the posterior part of the cloaca, and also 

 the rudiments of the two abdominal pockets (ab p). 



Fig. 2. Section through the cloacal region of an embryo belonging to stage P. 

 Zeiss A, ocul. 2. 



The figure shews the solid anterior extremity of the cloacal involution. 



Fig. 3. Longitudinal vertical section through the thyroid body in a stage between 

 O and P. Zeiss aa, ocul. i. 



The figure shews the solid thyroid body (///) connected in front with throat, and 

 terminating below the bulbus arteriosus. 



