F.XPLANATION OF PI.ATF.S 20, 21. 



//. Body cavity, .f r. Suprarenal body. ?/. Ureter, v ao. Ventral aorta (anterior 

 continuation of bulbils arteriosus). z>t'n. Ventricle. 7i'd. AVolftian duct. 



Figs. I a, \ l>, \c. 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 /' is a section through a point somewhat behind this close to the opening of the 

 Wolffian ducts into the cloaca. 



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

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



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 (M) connected in front with throat, and 

 terminating below tjie bulbus arteriosus. 



Fig. 4. Pancreas [pait) and adjoining part of the alimentary tract in longitudinal 

 section, from an embryo between stages L and M. Zeiss A, ocul. ^. 



Fig. 5. Portion of liver network of stage L. Zeiss C, ocul. 1. The .section is 

 intended to illustrate the fact that the tubules or cylinders of which the liver is 

 composed are hollow and not solid. Between the liver tubules are seen blood spaces 

 with distinct walls, and blood corpuscles in their interior. 



Fig. 6. Section through part of one of the suprarenal bodies of an adult Scyllium 

 hardened in chromic acid. Zeiss C, ocul. 1. The section shews the columnar cells 

 forming the cortex and the more polygonal cells of the medulla. 



Fig. 7. Transverse section through the anterior suprarenal body of an adult 

 Scyllium. Zeiss B, ocul. 2. Reduced one-third. The tissue of the suprarenal body 

 has not been filled in, but only the sympathetic ganglion cells which are seen to be 

 irregularly scattered through the substance of the body. The entrance of the nerve 

 («) is shewn, and indications are given of the distribution of the nerve-fibres. 



Fig. 8. Section through the sympathetic ganglion of a Scyllium embryo between 

 stages M and N, shewing the connecting trunk between the suprarenal body and the 

 spinal nerve (sp 11), and the appearance of an indication in the ganglion of a portion 

 more directly connected with the nerve. Zeiss D, ocul. ■2. 



Fig. 9. Section through one of the anterior sympathetic ganglia of an embryo of 

 stage Q, shewing its division into a true ganglionic portion {sy^), and a suprarenal 

 body {sr). Zeiss C, ocul. 2. 



EXPLANATION- (IF PLATES 20 AND 21. (X. p. 479.) 

 Complete List of Reference Letters. 



n mg: Accessory Malpighian body. caz'. Cardinal vein. ^t\ Germinal epithelium. 

 i: True kidney. /. c. Longitudinal canal of the Wojflian body connected with vasa 



