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



the uicsobliiist 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 coutinuous above with the 

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

 is developed. The remarkoble researches of Prof. W. K. Parker upou 

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

 ui)on 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 {Alona) as we find in the salmon. Nor is the rostral portion of 

 the basis of the skull nearly so precociously develo])ed; the supraor- 

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

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

 skull iu 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 tin rays at the time 

 of hatching, while iu Gadus they are ev^en 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, iu 

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

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

 the embryos of Gadus and Alo.m of the same stage. In Gambusia again, 

 in conformity with the generally accelerated condition of the dev'elop- 

 meut 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 intermaxillarj' rudiment, supraorbital bars, branchial 

 and hyoidean apparatus have reached a stage much uwre lully differ- 

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

 of Alosa, for example, 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 j unction 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 thio 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 exceptions, mainly amongst Lophobranchs. 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 



