Development of the Food-Fishes. 455 



living embryo, though by the fifth or sixth day after emer- 

 gence it is very movable and is raised and depressed con- 

 stantly, even before the oral aperture exists. Behind the 

 second arch the four successive branchial arches are seen as 

 stout cartilaginous rods developed in the anterior margin of 

 each cleft. By a forward movement of the lower curved 

 rami of these arches they become approximated, so that a 

 transverse section, if very slightly oblique, may pass through 

 the series with the exception of the first arch. 



Fins. 



At a very early stage a fold of epiblast in the post-otocystic 

 region is the commencement of the pectoral fin. This thick- 

 ened fold upon each side assumes a rude oval outline and lies 

 in a liorizontal plane upon the yolk. The proximal portion 

 becomes narrowed and much denser, due to the median intru- 

 sion of mesoblast tissue, which pushes its way between the 

 upper and lower epiblast cells of the fin-fold, and, ceasing 

 before reaching the limits of the fin, gives the margin a more 

 transparent appearance. When the embryo emerges, the fin 

 is a stout fan-like structure and has shifted slightly from the 

 original horizontal plane. The basal thickening still con- 

 tinues, and the mesoblast cells contributing to it show a ten- 

 dency to assume a radial arrangement, these radial lines 

 extending also into the thin distal border, while at the centre 

 of the peduncle cartilage develops independently, and ex- 

 tends distally as a thin central plate, unconnected at the base 

 with any pectoral arch rudiment. Whether this central 'car- 

 tilage develops into the ossa hasalia^ the sole remnant of the 

 primitive fin of fishes, or breaks up into radial rods, was not 

 made out. 



Cranium. 



Simultaneously the first skeletal elements of the cranium, 

 until now a mere fibro- membranous investment of the brain, 

 appear as two cartilaginous bars, the trabecula3, interesting as 

 showing at once longitudinal bifidity in the embryo. In the 

 interspace between them the hypophysis passes down to meet 

 the pituitary diverticulum. The roof of the oral aperture is 

 pushed up, not only at the point where the infundibulum is 

 formed, but along the middle line anteriorly, this median 

 involution giving the mouth in cross section a deeply-grooved 

 character, with a flattened base or floor. At the oral end of 

 the notochord and along each side two dense plates of carti- 

 lage arise — the parachordals — which grow rapidly and form 



31* 



