Note on the Pectoral Fin of the Sole, Achirus capensis. 105 
There are many illustrations in the body of how ready the mesoderm is 
to collaborate with any process initiated by the ectoderm or endoderm, by con- 
tributions of muscular or supporting tissues. Examples of this are abundant, 
for instance, in the elaboration of the nervous system and the sense organs, 
or, to take a more kindred example, in the provision of the supporting radials 
for the vertical or unpaired fins. In fact this collaboration and assistance 
is sometimes carried so far that the mesodermal elements usurp the place 
and function of the original organs entirely, as, for instance, in the case of 
the notochord initiated by the endoderm, and perhaps in the apparently 
primitive pectoral of the Dipnoi initiated by the ectoderm. 
The question, therefore, whether or not the skeletal elements of paired fins 
are derived from the visceral arches or arise as in the case of unpaired fins 
may be looked upon as supplementary to the question of the origin of fins as 
such, the origin of radials and girdles being another though interesting and 
important enough question in itself, particularly in connection with the 
possible derivation of the skeleton of the pentadactyle limb from that of the 
fish limb. 
If, therefore, we must look to some epidermal structures or superficial 
modifications of the body as the precursors of paired limbs, we have — (1) 
external gills whose primary function is respiration, (2) a problematic longi- 
tudinal fold of the body with the function, also problematic, of balancing the 
body, (3) vertical folds of the body, whose function is primarily connected 
with the mechanism of respiration by gill slits. 
With regard to the external gill theory, it may be helpful to keep in view 
the probable meaning of the external gills or other additional organs of 
respiration, which appear to have displaced the simple external respiratory 
organs so characteristic of animals devoid of gill slits. The most obvious 
significance of the appearance of these simple organs is that the supply of 
water to the gill slits has in some way or other become partially cut off. 
This may occur under various altered conditions of environment, as, for 
instance, in a prolonged embryonic stage, rendered possible by a more 
abundant supply of food in the form of yolk. A protecting shell is present 
in such cases, and this, of course, seriously interferes with the free access of 
water. In some such cases the yolk and body may become highly vascularised 
as in the large egg of Galeichthys feliceps, which is carried about in the mouth- 
cavity of the parent and thus has the benefit of its respiratory apparatus, 
or the egg of the Cape Bellostoma in the shell of which there are numerous 
slit-like apertures. In the pelagic egg of a Cape fish the pectoral fin was 
observed to be in constant motion — yet another device connected with 
respiration. Compare the branchial character of the paired limbs in 
Ceratodus. 
In other cases, more particularly the embryonic Elasmobranchs, more 
specialised supplementary respiratory organs are developed in the form of 
