720 PROFESSOR W. C. M'lNTOSH AND MR E. E. PRINCE ON 



neither a primary nor a later endogenously-formed yolk-nucleus has been made out in 

 the vitellus of the Teleostei. Upon this vexed question centres the interpretation of 

 the trophic part of the ovum. 



That the periblast-nuclei are really autoplastic, would seem to be the conclusion most 

 agreeable to the facts of the case,* and if the yolk were ancestrally divided into separate 

 nucleated masses or cells, as was most probably the case, then upon the breaking down of 

 these yolk-segments, to form the existing syncytium of the Teleostean ovum, the nuclear 

 matter would likewise become diffuse. It is possible, therefore, to look upon the peri- 

 blastic nuclei as the revival (segregation) of the primary nuclear bodies. The vitellus in 

 one species (Temnodon saltator), described by Agassiz and Whitman (No. 2, p. 14), 

 still shows the division into large yolk-segments without nuclei, though the segmenta- 

 tion is not total, a large central mass remaining uncleft. These large segments are 

 much flattened, and appear beneath the marginal periblast, with which, during epiboly, 

 they progress round the central yolk-nodule towards the vegetal pole. A similar con- 

 dition occurs in the pelagic egg of the sole (PL XXII. fig. l), in which a series of vesicles 

 or segments appear under the disc in the lenticular stage, and spread with the blastoderm 

 so as to form a superficial layer over the entire yolk. In the extremely pellucid egg of 

 the sprat, again, the whole yolk is imperfectly divided into a series of polyhedral masses. 



Even holding to the position that the cell is essentially of a uninuclear character, no 

 difficulty is presented by the multinucleate periblast, for each may be regarded as the 

 centre of a cell whose outline is undefined. It must be granted also that little difficulty 

 is presented to those who regard the yolk as a single cell — if, as Butschli holds, a single 

 cell during proliferation may exhibit all the gradations from a uninuclear to a multinuclear 

 condition, and from the latter retrogress to the former condition without once forfeiting 

 its character as a single cell. On the other hand, the syncytium, as Haeckel conceives 

 it, though formed of cells originally separate, and including therefore many nuclei, is 

 still a cell. 



There are many appearances in the living ovum which indicate that the periblast 

 contributes cells to the blastoderm, such cells being segmented extra-embryonically.t 

 This point belongs to a later stage of development, and we can here merely make a 

 reference to this segmentation of the periblast in its bearing upon the real significance of 

 this layer. 



In an ovum of Gadus ceglefinus, at the close of the first day after fertilisation, the 

 nuclear zone was well marked, and the homogeneous protoplasm composing it rose into 

 minute prominences or depressed conical papillae, upon each of which a nucleus appeared 

 to be seated (PI. II. figs. 4 and 4a, n). This botryoidal appearance was unmistakable, 



* Tt would not be accurate to speak of these nuclei as genuine " autoplasts," for these latter bodies never become 

 the centres of cells produced by cleavage. It is essential to the autoplast that the surrounding matrix remains unseg- 

 mented. 



t The growth of the blastoderm by marginal conversion of cells is a phenomenon that continued investigation shows 

 to be widespread ; it occurs in many Invertebrates — in Cyclostomes, and, as Balfour and Deighton unmistakably demon- 

 strated, in Birds. Vide "Renewed Study of the Germinal Layers of the Chick" {Quart. Jour. Micr. Sci., xxii. p. 177). 



