DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 747 



chordal sheath, and Balfour refers to both layers as closely adherent, though distinct, but 

 the former apparently decreases in thickness, and is then difficult to see (No. 15, vol. xi. 

 p. 421). From the mesoblastic perichordal sheath alone the vertebral bodies originate, 

 while its outer limiting stratum (the elastica externa) gives origin to the arches. The 

 neural arches precede the haemal in development ; no trace, in fact, of the ventral 

 processes being discernible when the neural arches project some distance dorsally. 



Of course, in a degenerate skeletogenous layer, such as the Teleostean perichordal 

 sheath, the identification of the precise layers, seen more favourably in other fishes, is 

 attended with much difficulty ; and one of us, in attempting to distinguish the different 

 laminae, has referred to the outer layer as a " limitans externa " (No. 122, p. 454) ; indeed, 

 the opinion expressed that the existence of an " elastica externa " in Teleosts, is a doubt- 

 ful point, is supported by the fact that such a membrane does not properly exist in 

 Amphibians, as well as in the Amniota. Favourable sections of Teleostean embryos, 

 especially such a form as Cyclopterus, bear out, however, the above interpretation, the 

 external layer being very distinct. Outside the perichordal sheath itself in post-embryonic 

 stages plates of spicular substance develop. Thus in a young but mature specimen of 

 Pleuronectes, the oral end of the notochord is seen to have acquired such a spicular 

 sheath — formed apparently in the connective tissue outside the external limiting 

 membrane — a distinct interspace separating the plate from the perichordal sheath. Four 

 rami of the same chitinous substance project, one pair dorsally and one pair ventrally, 

 and are well seen in sections through the otocystic region. 



Branchial System. — The head of the Teleostean embryo consists, as already indi- 

 cated, of an expanded mass, chiefly neurochordal, or rather brain-tissue, and separated 

 from the cortex of the yolk below by a thin layer of hypoblast (hy, PI. III. fig. 1). The 

 hypoblast forms here the roof of the sub-oral cavity, which has no floor, or rather, its floor 

 is simply the periblast enveloping the yolk. Behind and below the ears a large oval area 

 is apparently pushed in, resulting in the perforation of the lateral epiblast on each side of 

 the otocystic region, these fenestrae (poa) communicating with the primitive mouth- 

 chamber within (PI. VIII. figs. 3, 4). This opening, which may be called a primitive 

 opercular opening (poa), though the true operculum is a new and later growth, is plairly 

 visible in Molva vulgaris on the fourth day, along with a number of superficial irregu- 

 larities, doubtless connected with the active changes going on at this point in connection 

 with the branchial arches (PI. X. fig. 6). The significance and function of this cleft 

 (Spritzloch) upon each side is not readily understood, as the oesophageal lumen is not 

 apparently open in front, and any perivitelline fluid which gains access to the sub-cephalic 

 chamber, probably cannot find passage into the alimentary canal. Hoffman, however, 

 speaks of it as produced by an evagination of the oesophagus, at first below the otocyst, 

 but shifting forward and opening in front of the ear (No. 69, p. 7 ; vide his pi. i. fig. 5, 

 emb. sp., also fig. 3, on p. 7). These embryonic " Spritzlocher," he says, are merely 

 transient structures, and the ' interesting question is raised as to whether they may be a 

 reminiscence of the outer or extra-branchial system of the Cyclostomes, of which traces 



