[67] EMBRYOGEAPHY OF OSSEOUS FISHES. 521 



which was at first outermost is now anterior, and the face of the same 

 which was innermost is now posterior. The displacement of the whole 

 fin forward is not as real as would at first appear from our figures. 

 The growth of the head and the elevation of the body have effected such 

 changes in the relations of all the surrounding structures that the breast 

 fins have not escaped its influence, and while it is unquestionably true 

 that the breast fin has rotated on its base for an extent of almost ninety 

 degrees, part of the apparent change of position is undoubtedly due to 

 the concurrent development and increase in bulk of adjacent structures. 

 The great gains in bulk which have taken place in the brain and body 

 have had much to do with this alteration of the relative positions of ad- 

 jacent organs. 



The comparative embryology of the breast fins is very interesting, in 

 that some variation in its relative position is evident upon studying a 

 number of genera belonging to different families. In Cyhkim and Par- 

 ephippus the primitive pectoral folds appear very far back or behind 

 the vertical of the middle of the yelk- sack; in every other form with 

 which I am familiar they appear farther forwards. In Cyhium as many 

 as twelve muscular somites may intervene between the point of origin 

 of the breast fin and the auditory vesicle; in other forms the number 

 of intervening muscular somites is usually less, being sometimes re- 

 duced to two or three [Alosa and Pomolohus). The homodynamic rela- 

 tions of the pectorals would therefore seem to vary greatly in the larval 

 stages of Teleostei, and their serial relations to the gill arches are there- 

 fore also very variable. The unusual posterior origin of the pectoral 

 rudiments of Cyhium and Parephippus is also an indication that we may 

 expect to find other anomalous modes of develoj^ment, as indeed has 

 been the case with some of the forms studied by Prof. Agassiz — Lophius, 

 for example. 



As to the development of the ventral or pelvic pair of fins I have ob- 

 served little that is new, and can only call attention to the contrast in 

 the development of the organs as observed in Gambusia and Salmo. In 

 the latter the ventral fin-fold appears on either side about the time of 

 hatching, a little way behind the yelk-sack, with its base horizontal, 

 like the pectoral at first, and on a level with the lower wall of the in- 

 testine and just above the origin of the pre-anal* median natatory fold. 

 In Gambusia it grows out as a little papilla, and not as a fold, where 

 the body walls join the hinder upper portion of the yelk-sack a very lit- 

 tle way in front of the vent. These two modes of origin are therefore 

 in striking contrast and well calculated to impress us with a sense of 



* Under the head of the median fins I find that inadvertently nothing has been said 

 of the pre-anal. It is, however, developed in many embryo fishes, as in Alosa and 

 Pomolohus, to the greatest extent, less so in Salmo and Corcgonus, slightly in the later 

 stages of Cybium, Morone, and Parephippus ; it is wanting in Gambusia, Cottus, Apeltes, 

 Idus, Carassius, Tylostirus, Sij)hostoma, and Hippocampus, and is absent in Gadus on 

 acconnt of the peculiar mode of termination of the intestine. It is also present in 

 the larva of Lepidosteus, according to Agassiz. 



