614 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [60] 



15. — Development of the ribs. 



The frame- work of the trunk in Teleostei varies greatly in character; 

 the heads of the ribs also vary in different forms in relation to their 

 positions with respect to the vertebral axis and the notochord itself, 

 and it is on this account that it will not be possible to frame a general 

 theory of their development from the study of any one form. In some 

 the heads of the ribs articulate with the haemal processes ; in others 

 some distance below the vertebral axis, or directly with the sides of 

 the latter. All that I propose to do at present is to record what I 

 have observed in relation to their development in a single type not 

 hitherto the subject of embryological investigation. In this case the 

 relation of these skeletal appendages to the notochordal axis was so 

 intimate that their discussion may appropriately follow that of the 

 notochord itself. 



The form in question in which I have observed some of their stages 

 of development is Gambusia patruelis of Baird and Girard, which, as 

 already remarked, tends to develop its skeletal frame work very pre- 

 cociously as cartilage, even long before the complete absorption of the 

 yelk-sack, which is an unusual feature, and one to be accounted for 

 probably by the fact that this species, like Cyprinodonts generally, 

 develops its young within the ovary ^iviparously to a remarkable de- 

 gree of advancement. The embryos used were in an advanced state of 

 gestation in the ovarian follicles, from which they were removed and 

 cut into transverse and longitudinal vertical sections. 



In this genus the cartilaginous rudiments of the ribs were found to 

 abut directly against the notochord at its side and above the middle 

 of the side, where the head was somewhat larger in circumference 

 than the distal portions. At the hinder portions of the body cavity 

 their origins were somewhat more ventral than in the middle and ante- 

 rior regions. They arise in pairs and extend obliquely outward, back- 

 wards, and downwards between the muscular and splanchnopleural lay- 

 ers, following the intermuscular septa as perfectly cylindrical rods, and 

 appear to be surrounded by a stratum of connective tissue, which is 

 continuous with that surrounding the notochord, and in which presum- 

 ably the ossification of the vertebral bodies and the first superficial 

 sheath of bone of the ribs themselves will take place at a later i)eriod. 

 From their origin at the sides of the notochord, in the middle region of 

 the body, they bend downwards and follow the courses of the septa be- 

 tween the muscular plates, just where these terminate on the splanch- 

 nopleure. The foregoing describes fairly their relations to the surround- 

 ing tissues, but their finer structure is somewhat remarkable and calls 

 for special notice. They do not present the appearance of cartilage as 

 seen in the cartilaginous rods of the branchial arches of the same em- 

 bryo, nor that of the parachordal plates or trabecular cartilage of the 

 base of the skull. They, in fact, recall nothing of the structure of any 



