748 PROFESSOR W. C. M'INTOSH 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 (au, PI. VIII. fig. 4), for in pelagic forms, the shifting 

 of which Hoffman 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, 

 PI. XL 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 (PI. 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, 

 PI. XI. fig. 8). The mesoblast thus intruding into the oral hypoblast becomes columnar, 

 and forms paired rod-like masses (PI. XL 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 (i.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 (PI. VIII. fig. 8) as pale 

 structureless bands, with intervening cellular tissue, and passing transversely towards the 

 mesial ventral line beneath the otocysts. Balfour 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. Vogt shows, in an embryo of 

 Coregonus palcea, 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 XL 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 oesophagus, 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 hypoblastic 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 



