DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 727 



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 Haeckelhi 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 Kupffer 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. 243). 



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, i.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 Scott and Osborn found in Triton that the two bilateral masses were invaginated 

 as such, and were never confluent in the middle liue, the axial epiblast and hypoblast being 

 only in contact along that line (No. 147, p. 455). Scott 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 Shipley'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). Balfour 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 Balfour and Parker give no account of the origin 

 of the mesoblast. In certain Teleosteans, Haeckel describes a bilateral development 

 (Jenaische Zeit., Bd. ix.), while Kowalewsky 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 

 (PI. II. fig. 15<x), is in close apposition to the epiblast, ep, above. In the middle line 



* " Gastraea Theorie," see Quart. Jour. Micr. Sci., vol. xiv., note on p. 32. 



t 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. 



