284 EMBRYOLOGY OF THE LOWER VERTEBRATES cm. 



completes its development in the way described. The whole series 

 extends through about 25 segments but of these only about the 

 posterior half take part in the formation of the interrenal rod : the 

 anterior ones either atrophy completely or develop into small 

 accessory interrenals. 



The chromophile organs of the Elasmobranch (Swale Vincent, 

 1897) are small, rounded, segmentally arranged bodies lying ventral 

 to the intercostal arteries — the anterior few on either side forming 

 a continuous structure which was regarded by the earlier workers as 

 an accessory heart (Duvernoy). These bodies are, as Balfour showed 

 (1878), derivatives of the sympathetic ganglia. In a Scyllium of 

 about 53 mm. the lateral part of the ganglion rudiment begins to 

 show differentiation from the rest, its cells being relatively smaller 

 than those which are destined to become ganglion-cells, and their 

 protoplasm not only staining more deeply with ordinary stains but 

 also developing the characteristic chromic acid reaction. In the 

 Scyllium of 90 mm. the chromophile organ has assumed its definitive 

 rounded form. Intrusive connective tissue forms a sparse stroma 

 and capsule and in the former a capillary network is present. The 

 series of segmentally arranged chromophile masses undergoes much 

 modification in subsequent development — some, particularly at the 

 ends of the series, aborting, others undergoing fusion. The details 

 vary in different genera, the result being a striking variety in the 

 adult arrangements in the various members of the group. 



Amphibia. — Brauer (1902) in his work on the renal organs of 

 Hypogeophis gives a clear account of the development of the supra- 

 renals. 



The interrenals appear as in Elasmobranchs in the form of 

 cellular proliferations of the coelomic epithelium, in this case a 

 little external to the root of the mesentery. These proliferations 

 are paired and segmental in their arrangement, and extend from 

 the region of the pronephros - to that of the cloaca. The cellular 

 buds become constricted off from the coelomic epithelium and he 

 above it as rounded masses embedded in the mesenchyme. As the 

 two posterior cardinal veins approach and fuse the interrenal buds 

 become displaced upwards so as to lie between the cardinal vein 

 and the dorsal aorta. As development goes on processes of fusion 

 take place between the rudiments more especially anteriorly where 

 they come to form an unpaired elongated mass lying below the 

 dorsal aorta and for the most part dorsal to the posterior vena cava 

 (i.e. the fused posterior cardinals) but here and there passing 

 laterally round the vein .to its ventral surface or even piercing it— 

 the fusion between the two cardinals having been obstructed at 

 such points. In the posterior half of the organ the several rudiments 

 retain their distinctness and he on the ventral face of the opistho- 

 nephros. 



The chromophile bodies develop as in the Elasmobranchs from 

 split off portions of the sympathetic ganglion rudiments. These 



