DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. Ti 
visible, and is inflected as the peripheral rim when barely one-tenth of the vitellus is 
covered, whereas fully a sixth is enveloped before the expansion of the shield is indicated. 
When it first appears the outer layer is distinguishable only by the slightly depressed 
appearance of its cells. It is a single layer, and is difficult to make out, as it does not 
present the regular disposition or columnar character of the ectoderm in other forms. 
The second stratum is well marked when the blastoderm extends over a quadrant, and, 
as already pointed out, its cells are not at all depressed, but are rounded or polygonal, and 
form several layers—indeed, they are distinctly marked off from the corneous layer. The 
existence of this layer has been disputed by HarckEt in these words—* I do not consider 
the idea of a special nervous layer many embryologists separate from the cuticular 
sensory layer to be confirmed ;”* and Kuprrer denies that this layer exists laterally, 
for he distinguishes the corneous stratum only, and indeed doubts the presence of a 
median sensory layer as such, the outer epiblast appearing to him to merge in the neuro- 
chordal mass below, as though it alone gave origin to it (op. cit., p. 248). 
Mesoblast.—The origin of the mesoblast is still a point affording matter for discussion, 
but the Teleostean blastoderm, it may be readily surmised, does not offer great facility 
for deciding the matter.t That it is not a primitive layer, but is derived from one of 
the primary layers, 7.e., ectoderm or endoderm, is beyond dispute. 
LANKESTER seems to have been the first to suggest that, viewed phylogenetically, the 
mesoblast arose as a paired outgrowth of the entoderm, a fact which KowaLewsky had 
ascertained to be true for Sagitta (No. 85, p. 827). 
In the Mollusca and Annelida we know that the mesoblast usually arises not as a 
single sheet, but as two distinct masses, just as in Amphioxus and ‘many Craniates. 
Thus Scorr and Osporn found in Zriton that the two bilateral masses were invaginated 
as such, and were never confluent in the middle line, the axial epiblast and hypoblast being 
only in contact along that line (No. 147, p. 455). Scorr also affirms in Petromyzon that 
some mesoblast (dorsal) is invaginated with the cells of the mesenteron, while the cells of 
the ventral mesoblast are derived from the superficial cells of the yolk; but Sarpey’s later 
investigations have demonstrated that in this form no mesoblast is invaginated, the two 
longitudinal bands being differentiated in situ (No. 149, p. 244). Baxrour showed, and he 
is confirmed by His, that in Elasmobranchs the two bands arise in the manner just stated 
(No. 14, pp. 35-56); but in Lepidosteus BALFourR and PARKER give no account of the origin 
of the mesoblast. In certain Teleosteans, Harcket describes a bilateral development 
(Jenaische Zeit., Bd. ix.), while Kowatewsky says it originates from an invagination 
of the embryonic rim (No. 86). In speaking of the epiblast, it was indicated that our 
observations do not show such an inclusion of mesoblast by the reflected layer of the 
blastoporic lip ; and unlike the condition in Rana and other forms, the infolded layer, hyp 
(Pl. II. fig. 15), is in close apposition to the epiblast, ep, above. In the middle line 
* “Gastrea Theorie,” see Quart. Jour. Micr. Sct., vol. xiv., note on p. 32. 
+It need hardly be pointed out that in so familiar an ovum as that of Rana, the precise origin of the mesoblast is 
really undecided, and it is still to be settled whether the layer is derived from the “intermediary ” mass of small cells, or 
from the endoderm by proliferation, as seems more probable. 
