EXPLANATION OF PLATE XVIIL 



Complete list of reference letters. 

 Nervous System. 

 sp. n. spinal nerve. sy. g. sympathetic ganglion. n. nerve. 



Alime^^tary Canal. 

 cl. cloaca. in. cl. cloacal involution. oe. ep. oesophageal epithelium. 

 th. thyroid body. /?«;«. pancreas. 



General. 



pp. body cavity. ca. v. cardinal vein. cau. c. caudal vein. 



T. ao. ventral aorta (anterior continuation of bulbus arteriosus). 



aur. auricle. nen. ventricle. ic. d. Wolffian duct. o. d. oviduct. 



XI, ureter. ah. p. abdominal pocket (pore). p c. pericardium. 



m.m. muscles. s r. suprarenal body. /y. lymphoid tissue. 



Fig. \ a, \h, \ c. Three sections through the cloacal region of an 

 embryo belonging to stage 0. Fig. I ah the anterior of the three sections. 

 Zeiss A, ocul. 2, Reduced one-third. 



Fig. 1 a shews the cloacal involution at its deepest part abutting 

 on the cloacal section of the alimentary tract. 



Fig. 1 6 is a section through a point somewhat behind this close to the 

 opening of the Wolffian ducts into the cloaca. 



Fig. 1 c shews the opening to the exterior in the posterior part of the 

 cloaca, and also the rudiments of the two abdominal pockets {ah. 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 and P. Zeiss aa, ocul. L 



The figure shews the solid thyroid body {t/i.) connected in front with 

 throat, and terminating below the bulbus arteriosus. 



Fig. 4. Pancreas (j)a7i) and adjoining part of the aUmentary tract in 

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

 ocul. 2. 



Fig. 5. Portion of liver network of stage L. Zeiss. C ocul. 2. The 

 section is intended to iUustrate the fact that the tubules or cylinders of 

 which the Hver 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 Scylhum hardened in chromic acid. Zeiss C, ocul. 2. 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 (n) 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 {.9p. n.), 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 ganghonic portion {sg. g.j, 

 and a suprarenal body {s r). Zeiss C, ocul. 2. 



