748 PROFESSOR W. C. M‘IINTOSH AND MR E. E. PRINCE ON 
are observable in the Elasmobranchs. Each aperture, poa, has a strongly marked 
corrugated border or fold, which sweeps in a graceful curve round the opening, and 
passes forward beneath the otocysts (aw, Pl. VII. fig. 4), for in pelagic forms, the shifting 
of which Horrman speaks was not observed, and in front the fold is gradually lost. 
The opercular flap is a much later outgrowth from the tympanic region, apparently a 
fold of the integument, which protrudes, and grows backward over the gill-slits (ope, 
Pl. XI. figs. 10, 11). Below the hind-brain and otocysts, the hypoblast shows great 
increase in its cells, so that by the time the heart is defined it forms a thick supra-cardial 
plate (Pl. XI. figs. 2, 7, 8), beneath which mesoblastic cells make their way by a down- 
ward growth of the lateral cephalic masses. The sub-cephalic floor of hypoblast and 
mesoblast is limited below by a somewhat ill-defined layer of nucleated periblast (per, 
Pl. XI. fig. 8). The mesoblast thus intruding into the oral hypoblast becomes columnar, 
and forms paired rod-like masses (PI. XI. figs. 5, 6, 7). The cells are concentrically 
arranged along the axis of the transverse bars. LEREBOULLET evidently refers to the down- 
ward growth of mesoblast, and speaks of it as a ventral lamella (7.e., splanchnopleure), out 
of which, he adds, is formed later “the maxillary and hyoidean elements, and the gill- 
supports.” While the appearance of serial mesoblastic thickenings along the floor of the 
pharynx is a marked feature in Teleosteans some days before emerging from the egg, 
their disposition and conformation are very difficult to make out. There is indeed con- 
siderable variation in the condition of the branchial region, and this is especially seen in 
newly-hatched gurnards. Usually three branchial bars are visible (Pl. VIII. fig. 8) as pale 
structureless bands, with intervening cellular tissue, and passing transversely towards the 
mesial ventral line beneath the otocysts. Batrour and Parker (No. 18) noticed in 
Lepidosteus, six days after fertilisation, two transverse streaks on either side of the hind- 
brain, From a comparison with the sturgeon they judged them to be branchial clefts, but 
in section these clefts could not be detected. In the early condition of the branchial 
system the study of sections is by no means easy. C. Voer shows, in an embryo of 
Coregonus palea, thirty-six days old, branchial vessels, but indicates no skeletal bars (vide 
No, 155, Taf. ii. fig. 58). The fact seems to be that, soon after the arches are distinctly 
formed as definite bars, a vessel, or rather a long thread-like lacuna, is formed along the 
posterior margin of each bar (Pl. XI. figs. 9, 11). Five transverse bands, some- 
times an indication of a sixth, extend later on each side across the floor of the wide and 
flattened cesophagus, from a point just behind the eye to a little distance beyond the 
otocyst, so that the floor becomes raised into a series of cross-ridges which cease in the 
middle line, and between the ridges the hypoblast is pushed so that the mesoblastic 
ridges gradually become separated by hypoblastic septa. PARKER speaks of these ridges 
as separated by the dehiscence of the thinned interspaces between them (No. 117), but 
this hardly describes the process correctly, the rib-like thickenings being more truly 
separated by the paired hypoblastie diverticula or septa, these being pushed out from the 
sides and floor of the pharynx and affecting the differentiation of the serial gill-arches. 
Dehiscence takes place, it is true, but much later, and it results in the formation of 
