DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 729 
with the diminished neurochordal mass (vide No. 114, Taf. iii. figs. viii, and ix,) in that 
form. 
Further forward (Pl. IV. fig. 3) it apparently ceases altogether, the cells beneath the 
optic vesicles, op, being hypoblastic, while the denser stratum, ep, above, is neurodermal 
(sensory epiblast), unless the small strand of cells filling up the triangular fissure on each 
side be a continuation of the mesoblast behind (marked in the fig. mes ?). OELLACHER’S 
representation of this region is not unlike our fig. 3, Pl. IV., but here again mesoblastic 
cells are shown as somewhat abundant; his mesoblastic “ Kopfplatten” consisting of 
three or four layers, which continue laterally as flattened peritoneal plates. This latter 
structure is wholly absent in our forms, the marginal ale being simply epiblast and 
hypoblast, though the very minute group of cells mentioned (mes? Pl. IV. fig. 3) may 
represent OELLACHER’S cephalic mesoblast. Our figures (Pl. IV. figs. 3, 4, 16, and 16a) 
support the view that the mesoblast does not yet extend into the head-region, the 
cells at x and y being obviously neurodermal. If the foregoing conclusion be correct, 
the mesoblast arises for the most part in situ from the lower-layer cells in the trunk- 
region proper—that is, excluding the pre-otocystic and caudal portions—by a process not 
of delamination purely, but of mechanical separation, the intruding neurochordal cells 
from above actually pushing aside the subjacent cells as two longitudinal lateral plates. 
It is not easy to see why mesoblastic cells should, as appears to be the case, be absent 
so largely from the cephalic region. Their absence would be accounted for if the 
mesoblast be really a forward growth from the trunk-region, and most probably also from 
the posterior mass of indifferent cells. Such a forward growth has been regarded as the 
sole process of mesoblastic growth (KOLirkER, No. 81); and if in its differentiation the 
mesoblastic cells are separated at first just in front of the primitive streak, it will be dift- 
cult to show that some such process of forward growth is not involved. The cells, in fact, 
below the primary ectoderm form a median layer, when the rim is first invaginated below 
it, and since BaLrour and Dericuton find in the chick (No. 19, p. 180) that the main 
mass of the posterior indifferent cells (primitive streak) is really produced by epiblastie pro- 
liferation, it follows that some mesoblast is really indirectly of epiblastic origin. BAMBEKE, 
indeed, regards the mesoblast in Teleosteans as the lowest delaminated stratum of the 
primary upper layer of the germ (Von Baerr’s animal layer), ze., ectoderm. This upper 
layer in his view divides into three, viz., the corneous, neurodermal, and mesodermal 
layers (No. 20a, pl. iii. fig. 8, pp. 57-58). Delamination solely will not account for the 
fact that in Teleosteans the mesoblast is certainly best developed in the posterior region,* 
as would be implied by the theory of forward growth, and we see that it thins away 
anteriorly. A comparison of figs. 3, 4, and 5a—5c, PI. IV., sutticiently demonstrates this. 
Even at a later stage the same feature appears (see figs. 10 and 11, Pl IV.), as though 
the mesoblast in extending anteriorly into the head receives continual additions from 
behind. In Petromyzon, Sure.ey, indeed, regards the muscular elements of the mouth 
* This is also the condition in Elasmobranchs, the mesoblast being accumulated at the posterior end as prominent 
tail-buds (loc. cit., p. 557). 
VOL. XXXV. PART III. (NO. 19). 6A 
