1004 REPOKT OF COMMISSIONER OF FISH AND FISHERIES. [24] 



bariuouious conclusions regarding this subject. The researches of A. 

 Agassiz* upon the mode in which the tail of the most highly sjiecial- 

 ized fishes is developed are the most important and extensive, but it is 

 to be regretted that his illustrations do not give the outlines and rela- 

 tions of the hypural choudrilications in greater detail for all of the 

 species which he has considered, his object being apparently to identify 

 the forms and trace their metamorphoses without entering too much 

 into elaborate minor details. 



For the relations of the embryological structures involved we are 

 still mostly dependent upon the researches of I>alfour, Oellacher, Lotz, 

 Swirski, Dohrn, and myself. 



Since Balfour's discovery that the paired tins are apparently devel- 

 oped, at least in the Elasmobranchs, from continnons lateral folds of 

 the epiblast into which the mesoblast j^roliferated to form the vascular, 

 axial, radial, and muscular systems of the limbs, there has been a 

 large amount of evidence gathered which shows that the history of the 

 paired and unpaired fins of Teleosts is very similar. Oellachert was 

 the first to trace the origin of the axial substance of the pectoral limbs 

 of the Trout to the mesoblast. Since Oellacher's observations have been 

 published, Dohrn | has reconsidered the development of the paired and 

 uni)aired fins ontogenetically, and has fully confirmed and greatly ex- 

 tended Balfour's original conclusions, so that we now have a secure 

 foundation upon which to found a theory of the fins which does not rest 

 upon hypothetical assumptions, but upon observed facts. 



Dohrn holds essentially the following views respecting the origin of 

 the i>aired fins, viz : (1) As shown by Baltour, they are derive<l from 

 continuous, inferior, horizontal, lateral folds of epiblast, into which 

 muscular and other mesoblast proliferates. (2) The muscular somites 

 then throw out ventral buds which are thrust into the fold, in segmental 

 order, when they become constricted off from the somites, mb, Fig. 1>, 

 PI. X. (3) These segmental muscular diverticula then divide into dor- 

 sal and ventral processes, which give rise respectively to the inferior 

 and superior muscles of the actinosts and rays. (4) The actinophores 

 are then formed by a gradual chondrification of the loose mesoblastic 

 tissue between the divided diverticula of the muscular segments. (5) As 

 the paired fins become longer and morepedunculate, the primitive actino- 



"The development of Lepidosteus. Part I. Proc. Am. Acad. Arts and Sci.,XIII, 

 1878, 65-76, 5 pis. 



On the young stages of Osseous Fishes. I. Development of the tail, Proc. Am. Acad. 

 Arts and Sci.,XIII, 1877, 117-127, 2 pis. II. On the development of the Flounders, 

 Proc. Am. Acad. Arts and Sci., XIV, 1878, 1-25, 8 pis. III. On the young stages of 

 Osseous Fishes, Proc. Am. Acad. Arts and Sciences, XVII, 1882, 271-303, 20 pis. 



t Beitrage zur Entwickelungsgeschichte der Bachforelle. Vorliiufige Mittheilung. 

 Ber. d. nat.-med. Ver., Innsbruck, 1879, pp. 141-143. 



tMitth. zool. Sta. Neapel, 1884. V. Die paarigen und unpaarigen Flossen der 

 Selachier, pp. 161-189. 



