1016 REPOET OF COMMISSIONER OF FISH AND FISHERIES. [30] 



.fluid, SO that the development of the rays is similar to that of the chorda 

 dorsalis of the animal. They are probably of mesoblastic origin, while 

 the fine embryonic fin rays, theprotopterygian stage of other Vertebrates, 

 are interposed between the epiblast externally and more or less nieso- 

 blast internally. The development of the rays of the Marsipobranchs 

 offers more analogy with that of Amphiox^is than that of any of the other 

 ray-bearing Vertebrates. 



In most anatomical hand-books it is exi)licitly stated that the perma- 

 nent rays are integumentary in their origin. Since I have again taken 

 up the subject with the object of finding out the true state of the case 

 by the help of more refined methods of investigation, I have reached a 

 conclusion which, in some respects, is opposed to the one generally 

 accejited. My sections of Salmon embryos show that while the embry- 

 onic rays are at first interposed in a single layer between the epiblast 

 and mesoblast which has wandered outward into the epiblastic fin-fold, 

 at a late period relatively these embryonic rays become invested by the 

 cells of the lower layer of the integument formed from the mesoblast, 

 seeming to sink into the latter as its cells multiply and surround these 

 rays externally. But the phenomena which occur prior to the forma- 

 tion of the embryonic rays themselves are even more conclusive as show- 

 ing that the larval integument or epiblast has little or nothing to do 

 with the formation of the permanent rays. 



If sections through the tail of an embryo fish are i)repared at about 

 the time that the lophocercal stage is almost fully developed, of Lophim^ 

 Belone, Fierasfer, and Alosa, for example, it will be found that the ver- 

 tical fin-folds are filled with an homogeneous substance, as pointed out 

 by Emery, which shows no traces of cellular structure, and contains no 

 cells whatever, except sometimes a few in the basal part of the fold and 

 near the axial mesoblast (r,ee Fig. 10, PI. X). Such cells evidently have 

 wandered outwards. In the tail, however, where the embryonic fin-rays 

 first appear, there is already an abundance of mesoblast, on either side 

 of which the layer from which the embryonic rays are formed is placed. 

 This layer seems at first to be very thin, as in Fig. 7/<, PI. X, and a 

 product of mesoblastic secretion, in part at least, even should it be 

 shown that what I have called pterygoblasts in the tail of the embryo Cod 

 have no real genetic relation to the embryonic rays, which I am strongly 

 inclined to think they must really sustain, on account of their form and 

 relations to surrounding tissues. Fig. 3, PI. IX. 



The embryonic rays are therefore clearly dependent upon the meso- 

 blast for their development, shico it can be demonstrated beyoud any 

 doubt that as fast as they appear in the more anterior i)ortions of the 

 vertical fin-fold their advent is preceded or accompanied by the out- 

 growth* of mesoblastic cells into the latter. The layer from whence the 

 embryonic rays are formed seems almost homogeneous, and only as 

 growth proceeds does it seem that tlie filamentous embryonic rays are 

 differentiated from it. The pterygoblasts first disappear at the basal 



