[65] EMBRYOGRAPHY OF OSSEOUS FISHES. 519 



The mesoblast, from which the median system of fin rays of the larvaa 

 of osseous fishes is developed, appears to be an outgrowth from between 

 the mesial, dorsal, and ventral points of meeting of the muscle plates, 

 and that it is pushed out into the natatory fold during the development 

 of the skeletal and muscular elements of these fins, and that it is con- 

 tinuous dorsad of the spinal chord, and ventrad of the notochord, with 

 the tract of tissue, from which the iuterspinous elements of the skeleton 

 are differentiated. It is, therefore, a part of that mesoblastic tract from 

 which the haemal and neural arches, interposed between the dorso- 

 lateral and ventro-lateral plates of muscle-segments, are differentiated, 

 and was primitively continuous with it. 



18. — The development of the paired fins. 



The paired fins of Teleostei, like the limbs of the higher vertebrata, 

 arise locally, not as blunt processes, however, but as short longitudinal 

 folds, with perhaps a few exceptions. The pectorals of Lepidosteus 

 originate in the same way. Of the paired fins, the pectoral or anterior 

 pair seems to be the first to be developed ; the ventral or pelvic pair 

 often not making their appearance until after the absorption of the 

 yelk-sack has been completed, in other cases before that event, as in 

 Sahno and Gamhusia. The ventral undergoes less alteration of position 

 during its evolution than the pectoral pair. 



In that the development of the pectoral or breast fins of Gadus is 

 typical of the group we can do no better than describe their evolution 

 in that form, as observed prior to and after hatching. The date of ap- 

 pearance of the first sign of the pectoral fin-fold varies somewhat in 

 different genera, but in Gadus it appears as a slight longitudinal ele- 

 vation of the skin on either side of the body of the embryo a little way 

 behind the auditory vesicles, as shown in Figs. 30, 32, 33, and 34, at ff, 

 and shortly after the tail of the embryo begins to bud out. At the very 

 first it appears to be merely a dermal fold, and, in some forms, a layer 

 of cells extends out underneath it from the sides of the body, but does not 

 ascend into it. It begins to develop as a very low fold, hardly notice- 

 able, and as growth proceeds its base does not expand antero-posteriorly, 

 but tends rather to become narrowed so that it has a pedunculated 

 form, as in Fig. 40. With the progress of this process, the margin of 

 the fin-fold also becomes thinner at its distal border, and at the basal 

 part mesodermal cells make their appearance more notably within the 

 second or inner contour line of If^ Fig. 40. In some species I am quite 

 well assured that there is at an early period a mesodermal tract or plate 

 of cells developed just behind the auditory vesicles, just outside the 

 muscle plates of this region, on either side, which may be regarded as 

 the source of the mesodermal cells which are carried up into the pectoral 

 fin-fold. This is developed at about the time of the closure of the blasto- 

 derm, and these lateral mesodermal tracts of tissue may be called the 

 pectoral plates. The free border of the fin-fold grows out laterally and 



