On tlic Inrval developmeiit of Amia calva. 649 



time enclosed the region of the heart, although their traiislucency 

 still permits the heart's outline to be seen. The principal chauge in 

 the fius is the increased growth of the pectorals; these have now a 

 stout muscular base and exhibit their first niovements. The caudal 

 vessels, c.v., are now less prominent on accouut of the eulargement and 

 thickening of the muscle-plates in the ventral region of the trunk; 

 a sub-intestinal vein, s. i., is noted. The anal opeuing is now obscured 

 by the longitudinal creasing at this point of the dermal fin ; it is now, 

 judging from surface view, that the pronephric ducts, p.n., establiS'h 

 their outer opening. 



Larva of the sixth day (PI. 10, Fig. 15; dorsal aspect in PI. 10, 

 Fig. 16; outline of distal end of sucking diso in PL 10, Fig. 15 a). 

 A general modelling of the head and yolk sac accompanies the 

 size growth of this stage. In the head the brain is becoming sur- 

 rounded by the cartilaginous cranium ; this growth, however, is not as 

 yet so complete as to obscure the profile of the fore-, mid- and hind- 

 brain, as noted in the figures of the larvae of fifth, fourth, third and 

 second days. The head contours are being fiUed in by the growth of 

 the rausculature of the opercula, gill arches and jaws. In the latter 

 notable advances are present; the membrane bones, premaxillary and 

 maxillary are forming, and give at once an Ämia-like appearance to 

 the young fish ; their establishraent has proceeded pari passu with 

 the reductiou in size of the sucking diso. Its distal outline at this 

 stage is shown in PI. 10, Fig. 15 a. The lower jaw has also the be- 

 ginning of its adult characters ; its increased size becomes more con- 

 spicuous by its constant movements in breathiug; the beginnings of 

 the jugular plate, jug., appear in the angle between its rami. A 

 general outlining of the thickened epithelial tracts of the mucous 

 canals can now be followed, especially in the temporal and supra- 

 orbital regions, where the closing over process so admirably described 

 by Allis is now taking place ; this process has already occurred along 

 the sides of the trunk and renders the lateral line less conspicuous 

 in surface view. The great reduction in the size of the yolk sac 

 gives the larva a tadpole-like appearance: around the sac's forward 

 end the enlarged opercula have become neatly modelled. There is a 

 noteworthy enlargement of the pectoral fin both in basal and dermal 

 parts; it has at present somewhat the appearance of a shark's fin, a 

 monoserial archipterygium, the dermal margin having made little ad- 

 vance is surroundiug the hindward projecting fin axis. 



