82 PAIRED FINS 



actual migration of the ready-formed material from one segment on 

 to another, but may be said to be due to * transposition ' from 

 one set of segments to another set up or down the series. 



In the Teleosts, where within comparatively recent times the 

 pelvic fins have shifted from an abdominal to a thoracic, and from 

 a thoracic to a jugular position, the nerve-supply is correspondingly 

 modified (Guitel [187]). In the last instance it may even be 

 drawn to some extent from a segment supplying the pectoral fin 

 (Lepidoleprus [Stannius, 416], Uranoscopus [von Jehring, 245]). 1 



In spite of the fact that the muscles in the fins of fish are not 

 always attached to the cartilages, yet there is no reason to believe 

 that the mesenchymatous mesoblast from which the latter are 

 developed is not derived from the same segments. We conclude 

 that the limbs are always developed from the segments correspond- 

 ing to their position in the adult, as indicated by the nerve- 

 supply. Change of position is not brought about by migration, 

 but by progressive growth at one end accompanied by reduction at 

 the other. Migration is apparent, not real. 



Finally, with regard to the origin of the paired fins, it may be 

 concluded that the position of the girdles in the body-wall, the 

 perforation of these girdles by a number of nerves supplying the 

 fins, the structure of the endoskeleton of the fins, the derivation of 

 their musculature and nerve-supply from a large and varying 

 number of segments, above all, the remarkable resemblances 

 between the development and adult structure of the paired and 

 unpaired fins, and the presence in both of exactly similar dermal 

 fin-rays (p. 212), that these and other facts mentioned above are 

 strong evidence for the lateral-fold theory, and receive their natural 

 explanation from it. 



In the Gnathostomata, the nasal sacs and the nostrils are dis- 

 tinctly paired. They do not come into connection with the 

 hypophysis. The latter (Fig. 10) is always small and develops 

 as an ingrowth of the ectoderm, on the roof of the buccal cavity, 

 which grows towards the infundibulum. As a rule, it becomes 

 nipped off, and converted into a glandular mass, the pituitary 

 body, lying inside the cranial cavity, attached to the infundibulum. 



The thyroid loses all trace of an endostylar structure, and 

 forms a glandular mass. A large vascular organ, the spleen, is 

 always present, situated near the stomach. The latter organ is 

 generally well differentiated and U-shaped, and the intestine to 

 some extent coiled. A renal portal circulation is established in the 

 kidneys (p. 114). 



The ear has one horizontal and two vertical semicircular canals. 



1 An investigation into the development of these fins is much needed, and 

 would be sure to yield interesting results. 



