DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 751 



follow, only a portion undergoes this change, viz., that part of each bar nearest the 

 pharyngeal cavity, i.e., throughout the entire upper part of each. The rest of the bar 

 remains indifferent in structure until a tubular cavity is formed from end to end. This 

 tube at first is apparently single, but later is divided by a delicate septum into two tubes, 

 an upper arterial and a lower venous trunk. External to and below the haemal canals 

 the loose epithelial covering of the bar becomes nodulate, a double row of papillae pro- 

 jecting on the posterior and ventral side of each arch. The appearance of these gill- 

 rudiments is thus preceded by a considerable interval by the conversion of the arches 

 into cartilage, as Lereboullet observed in Perca (No. 93, p. 623), the same author noting 

 in that species the growth of the gill-tubercles from the soft cellular membrane covering 

 the gill-arches (see his plate iii. fig. 7). Moreover, he speaks of them as hollow (p. 627), 

 an appearance probably due to the intrusion of mesoblast into each papilla, which is thus 

 provided with a mesoblastic core and a hypoblastic epithelial covering. 



The formation of these branchial tubercles belongs, it may almost be said, to the first 

 post-larval stages, and their subsequent development into the branchial fringes of the 

 adult leads beyond the present limits.* 



The further development of the early cartilages may be easily followed in a large 

 chondral element such as the hyomandibular, or the massive mandible itself. The disc- 

 like chondroplasts which form a single column along the entire length of the bar (PI. IX. 

 fig. 5), slightly alter in form, becoming wedge-shaped when seen laterally, and lie over 

 each other in an alternate manner, as though about to separate into two rows, — sometimes, 

 indeed, a disc becomes thin in its median part, and divides, resulting in two wedge- 

 shaped chondroplasts. Thus the original single column of chondroplasts becomes broader, 

 and exhibits two or more rows (PI. IX. fig. 7). In the mandible this change affects the 

 upper or articular portion, while the anterior growing part, which continues to lengthen 

 until the cartilages of each side meet at the tip of the extended oral floor, still maintains 

 its simple columnar character, and consists of a single series of chondroplasts. In other 

 cases, as Pouchet noted (No. 119, p. 296), the chondroplasts towards the extremities 

 lose their disc-like form — becoming irregular in outline and mingling with the enveloping 

 tissue — just as in the limbs of young Amphibians. In the region of the joints, as in the 

 upper or articular portion of the mandible, the chondroplasts become irregular, numerous, 

 and disposed round the joint conformably to the contour of the articulating element 

 (PI. IX. fig. 7). The development of a crest is due to the protrusion of similar small 

 irregular chondroplasts, which grow out, row upon row, as a strong lamella. Pouchet 

 regards these new chondroplasts as not due so much to fission of existing chondro- 

 plasts as to development from the nuclei, so plentiful in the perichondrium or " tissu 

 generateur" which clothes the bars (No. 119, p. 298). These nuclei he describes as 

 crowded at the margin, and, as they pass inward, become separated by the intrusion of 

 the hyaline matrix. 



* In Esoz and Perca the yolk has decreased and its circulation has heen almost obliterated, according to 

 Lereboullet, when the gills are formed (No. 93, pp. 613, 627). 



