516 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [62] 



the mesoblast of the inner medullary portion of the fleshy branchial 

 arches ; the branchial blood vessels are formed in the outer and hinder 

 part of the same medullary tract, which is continuous above with the 

 same layer of tissue from which the cartilaginous basis of the cranium 

 is developed. The remarkable researches of Prof. W. K. Parker upon 

 the development of the skull of the salmon leave nothing to be desired 

 upon that group, but I am assured that many details of the process 

 still remain to be worked out for other forms. For instance, the palato- 

 pterygoid bar does not seem to develop relatively so early in other 

 forms (Alosa) as we find in the salmon. Nor is the rostral portion of 

 the basis of the skull nearly so precociously developed ; the supraor- 

 bital bar is also weaker and arises at a comparatively later stage in 

 Alosa. In fact this tendency to manifest a later development of the 

 skull in these types appears to be related to the general backwardness 

 of the condition of development of the median and paired fins. In 

 Alosa, for example, there is no sign of cartilaginous fin rays at the time 

 of hatching, while in Gadus they are even later in making their appear- 

 ance. In the salmon, on the other hand, evidences of rudiments of fin 

 rays have already made their appearance at this stage in the tail, in 

 the dorsal, anal, and pectoral fins, while the rudiments of the ventral 

 tins are prominently developed while there is still no sign of them in 

 the embryos of Gadus and Alosa of the same stage. In Gambnsia again, 

 in conformity with the generally accelerated condition of the develop- 

 ment of the skeleton of the embryo, the skull shows a like tendency to 

 be more fully formed at an early period. In it the cranial tegmen, lab- 

 ial cartilages, the intermaxillaiy rudiment, supraorbital bars, branchial 

 and hyoidean apparatus have reached a stage much m.ore iully differ- 

 entiated than in other forms of the same relative age. In the embryos 

 of Alosa, for exami)le, at the time of hatching, the basihyal and glosso- 

 hyal cartilages are still in the form of an unsegmented plate, while in 

 Gambusia they have been developed long before incubation is complete. 

 The Meckelian cartilages of the lower jaw, however, develop concur- 

 rently with the oral opening and grow in length as its gape increases. 

 The quadrate cartilage retains its solid junction with the metapterygoid 

 in the slow-developing forms above alluded to, just as in the salmon. 

 Conscious of having added but little that is new to this part of the 

 developmental history of osseous fishes, we leave this portion of the 

 subject for fuller treatment at some future time. 



17. — The development of the unpaired or median fins. 



The development of the unpaired fins from a median dorsal and ven- 

 tral natatory fold seems to be general amongst osseous fishes, with only 

 a few unimportant excei^tions, mainly amongst Lopbobranchs. In the 

 cod embryo the natatory fold here alluded to appears soon after the 

 tail buds out from the caudal plate. It is at first a low fold of the skin, 

 as at nj] Fig. 32, which extends over the end of the tail and forward on 



