1054 REPORT OF COMMISSIONER OF Fli^H AND FISHERIES. [74] 



will occnr, and where alternate compression and extension or even sun- 

 dering of the continuous axial, skeletogenous, tubular uiembrane must 

 occur, owing to the simultaneous contraction of an extensive series of 

 successive muscular segments, entailing a ])ronounced bending of the 

 whole body. In some such manner we are, it seems to the writer, bound 

 to infer that the segmentation of the axial skeleton of tlie vertebrate 

 body was initiated, for the reason that the segmentation of the muscu- 

 lar precedes that of the skeletal system — nay, that the mesoblastic my- 

 ogenous layer of tissue from which the muscular segments develop is 

 subdivided into segments even before it can be said that there is other- 

 wise the slightest evidence that their component cells will become mus- 

 cular fibers. 



These facts seem to me to strongly re-enforce the conclusions of Hert- 

 wig and Sedgwick that the origin of the structures which led to the 

 development of vertebral metamerism may be traced as far back as a 

 little above the gastrula, and it may even not be too bold a procedure 

 to assume that the lateral gut-pouches^ whence the muscular segments 

 of Vertebrates are probably always developed, have been derived in the 

 course of the progress of evolution from the folded enteric walls of a 

 diploblastic or two-layered ccelenterate ancestry, as held by the latter 

 and Dr. E. B. Wilson,* now that structural bilaterality may be predi- 

 cated of many Acthiosoa, as is distinctly shown by the investigations 

 of these authors and Milne ^Marshall. If such is the true explanation 

 of the origin of vertebral metamerism, as it seems we have many 

 weighty reasons for believing, it is only a step beyond this to apj)ly the 

 doctrine to the whole of the fin and limb skeleton of fishes — an expan- 

 sion which it has already practically attained through the ontogenetic 

 researches of Anton Dohrn upon the Elasmobranchs, already aHuded 

 to elsewhere, though the relation of the anterior muscular somites to 

 the pectoral limb in Salmonoids amongst Teleosts was first indicated by 

 another author.f 



Examining longitudinal vertical sections of embryo Teleosts, the ol)- 

 server is struck by the fact that the vertebral spines and interspinous 

 elements of the vertical fins originate almost exactly in the intermus- 

 cular septa which separate the muscular segments, while the muschv'^ 

 which move the fins are quite as clearly derived from the upper and 

 lower extremities of the successive muscular segments corresponding 

 in position with the successive interspinous intervals. In the Elasnio- 

 ' branch embryo the correspondence between the successive muscnlar 

 somites and the diverticula of their upper ends, which they push up- 

 wards on either side over the spinal canal into the base of the dorsal 

 fin, is even more clearly marked in sections of embryo Dog-fishes than 



* The mesenterial filaments of the Alcyonaria. Mitth. aus d. zool. Sta. zu Neapel, V, 

 1. Hft.. pp. ]-27, 18S4. 



t Jos. Oellacher: Eutwiclielungeiuiger Organe der Forelle. Ber. d, uat.-med. Ver., 

 Innsbruck, 1878, p. 141. 



