DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 729 



with the diminished neurochordal mass (vide No. 114, Taf. iii. figs. viii 4 and ix x ) in that 

 form. 



Further forward (PI. 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 alse 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 (Kolliker, No. 81) ; and if in its differentiation the 

 mesoblastic cells are separated at first just in front of the primitive streak, it will be diffi- 

 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 Balfour and Deighton find in the chick (No. 19, p. 180) that the main 

 mass of the posterior indifferent cells (primitive streak) is really produced by epiblastic 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 Baer's animal layer), i.e., ectoderm. This upper 

 layer in his view divides into three, viz., the corneous, neurodermal, and mesodermal 

 layers (No. 20a, pi. 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, PL IV., sufficiently 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, Shipley, 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). 6 A 



