Structure of the Short Sun-Fish. 191 



from which the pigmentary and tubercular layers of the skin 

 has already been developed, but from which the conversion 

 into true skin has not begun, and that of the peripheral bones 

 has been arrested.* 



From what I have now stated, it is evident that the interest 

 to be attached to this tissue consists in its purely embryonic 

 character. The general appearance of the cells, their nuclei 

 and nucleoli, the uniformity of the tissue in every part of the 

 animal, and its chemical composition, all indicate this character ; 

 and when taken in connection with the embryonic state of the 

 bony tissue, and the rudimentary condition of the muscular 

 system, forms a very interesting and important character in 

 the species, and probably in the order of fishes to which the 

 one under consideration belongs. 



In a teleological point of view it is important, as it points 

 to the existence of certain laws which regulate the develop- 

 ment of animal tissues, namely, first, In the organic series, 

 tissues as well as forms undergo progressive development; se- 

 condly, This progressive development of the tissues mag be re- 

 tarded, retaining their early embryonic condition in certain beings 

 in the series; and, thirdly, Tissue is subordinate to form.\ 

 A sun-fish, in fact, as well as other fishes of its order, is as 

 highly developed, in so far as regards form, as any in the class ; 

 in certain of its tissues it is still in the condition of an embryo. 



The second peculiarity to which I shall refer, is the form of 

 the caudal fin of the orthagoriscus. The naturalist is familiar 

 with its truncated shape, but the anatomist has not yet ascer- 

 tained the cause of the peculiarity of this part of the skele- 



* Meckel, Comparative Anatomy, French edition, torn v. p. 185. Ac- 

 cording to the observations of Dr Jacob, in the Dublin Philosophical 

 Journal, the cetacea have no dermis, except we consider, along with him, 

 that the blubber is the true skin distended with' oil. The subcutaneous 

 fat of the cetacea, however, differs from the gelatinous vesicular tissue of 

 the sun-fish in having no primitive cells in its constitution, consisting of 

 common fibrous tissue inclosing in its areolae fat or oil cells. It may, 

 nevertheless, be considered as a tissue in which some of the primitive cells 

 have been developed into fibrous tissue, while others have become filled 

 with oil. 



f For an exposition of this last law of organization, see Dr Martin 

 Barry's memoir on Embryology, 2d series, Phil. Trans. 1839. 



