1S89.1 bob [Rydci. 



fibres, or the young trematode -which has bored as a Cercaria into the 

 coriiim of a fish and there become encapsiiled. 



It will be seen later that these facts have a profound significance as 

 respects the genesis of the endoskeleton met with in many mollusks, as- 

 cidians, balanoglossus and vertebrates. 



In the vertebrates the first intimations of cartilage are met with in 

 BrancMostoma around the notochord, but not as definite bars. The gel- 

 atinoid matter for the matrix appears to be present around and between 

 the notochord and the muscle plates and in the epipleural folds, but no 

 definite loose cell aggregations have as yet wandered into it to constitute 

 true cartilage. No true cartilage, in fact, exists in BrancJiiostoma, only 

 the materials and possibilities of it. 



In marsipobranchs the neural and hisemal arches are developed around 

 the anterior part of the notochordal axis only, and these elements here 

 present the character of a tissue of cartilage cells embedded in a dense 

 fibrous matrix the origin of which is, however, to be traced to a homo- 

 geneous circumnotochordal gelatinoid matrix such as is seen investing the 

 notochord of Branchiostoma. 



The next step in advance is made through the chimseroids and stur- 

 geons, in which the cartilaginous arches are developed for nearly or quite 

 the entire length of the notochord. From this point onward the cartilage 

 begins to preponderate around the notochord, and as we rise in the verte- 

 brate scale the neural and htemal arches, where they abut against the noto- 

 chord, expand in all directions as flat disks, so as to form a more or less 

 complete investment around the notochord. Eventually in birds and 

 mammals, the cartilage precociously replaces the notochord, and it dwin- 

 dles or aborts in the embryo so that by the time the latter is hatched or 

 born, traces only of the notochord remain within and between the 

 centra. 



The axial intermuscular tissue in the extremities of higher types seems 

 to have given rise, in the same way as above described, to the cartilagin- 

 ous matrix of the limb-bones. In Branchiostoma traces of continuations 

 of the gelatinoid intermuscular substance is found in the epipleural folds. 

 In the true vertebrates cords of proliferated cells in the axis of the limb 

 (prochondral cells of Strasser), pour out or aggregate to themselves more 

 and more of the primitive intermuscular and intermembranous gelatinoid 

 matrix. Definite bars of cartilage so arise, at first unsegmented, repre- 

 senting the whole of the future osseous merites or segments of the limb. 

 At first not sharply circumscribed, such bars eventually become definitely 

 and sharply outlined and imbedded in a matrix of fibrous connective tis- 

 sue, which is the perichondrium or germ of the future periosteum, in case 

 future ossification occurs. 



With circumscription of the definitely formed bars of cartilage the exu- 

 dation of a still more dense deposit of homogeneous matter occurs through 

 the instrumentality of the perichondrium. This is invariably thickest at 

 the oldest part, or at the middle, of the shaft or diaphysis of cartilage 



