105G RKPORT OF COMMISSIONER OF FISH AND FISHERIES. [76j 



tliese and other elements tended to be formed in the septii between the 

 muscular somites. These septa are evidently "points of rest," so to 

 speak, where chondrification seems to be favored and where the pro- 

 chondral elements of Strasser* first become defined when a bone is to 

 be preformed m cartilage, or where the basement membrane of mem- 

 brane bones first appears. 



Passing' now to the end of the vertebral axis of fishes, we find the 

 same thing verified again. We find the hypural elements, the anterior 

 ones at least, presenting this intersegmental relation to the muscular 

 segments, and finally we discover that the proximal ends of the meso- 

 blastic thickenings which are sent out from the hypural bones towards 

 the margin of the fin-fold, in and about which the i)ernuinent rays are 

 developed, rest upon such traces of nuiscular septa as are not ol>l it- 

 erated at the time the permanent tail-fold is formed, Xor does this 

 metameric influence end here, for we iind that in the development of the 

 permanent rays a large number of filamentous embryonic fin-rays are 

 fused together by a material aiuilogous to perichondrium, which the 

 embryonic rays themselves simulate, in order to afl'ord the basis for the 

 ossification of the pernument rays. So far-reaching, therefore, is the 

 influence of this metamerism that we are actually enabled to trace its 

 effects even to the fonnation of the tin-rays. 



XIIT. — The mesenchyme of verteerated embryos. 



As Hertwighas shown that the connective tissue cells in certain forms 

 [Pseudoccelin) are split off from the walls of the cleavage (;avity of the 

 blastula or gastrula stage, we nuiy say that after the genesis of the 

 myotomes a somewhat similar sundering, partly proliferation, of con- 

 nective tissues from the myogenous and somatopleural tracts of the ver- 

 tebrate embryo occurs and gives rise to the indifferent tissues, blood, 

 cartilage, and, later, bone. The character of this indifferent tissue is 

 commonly spongy, and is late in developing, just in ])ro])ortion as the 

 skeleton is retarded in its growth or development. The indifferent 

 stellate cells of this layer, which forms a sort of matrix or envelope for 

 the other tissues, here become metamorphosed into cartilage; then 

 into fibro-membrane; at other points into the endothelium of vessels,. 

 &c., so that it gives rise not only to the hard supporting structures of 

 the organism, but also affords the materials for the construction of the 

 channels for the conduct of the functions of irrigation, respiration, 

 nutrition, secretion, and excretion. Its part in embryonic development 

 is thus evidently very imi)ortaut; it, in fact, forms the bond between 

 the other structures — not only affords the means for sustaining the latter 

 mechanically, but also physiologically. It develops secondarily after 

 the triploblastic stage has been passed over in the vertebrate embryo. 



* H. strasser: Ziir Eutwickelung der Extremitiitenknoclien bei Salaiuautleni uiid 

 Tritonen. Morpb. Jahrb., V, 1879. 



