704 PROFESSOR W. 0. M'INTOSH AND MR E. E. PRINCE ON 



though the view is held by many authorities (Van Beneden, No. 25, pp. 52, 53; Hoffman) 

 that segregation is equivalent to cleavage, and that when the disc is defined the ovum 

 consists of two cells — one being the germ, and the other the yolk. The behaviour 

 and undoubted function of the deutoplasmic globe is opposed to this view, the separation 

 of the germinal matter from the inert yolk being protracted and undefined, and wholly 

 unlike cleavage. Nor in the syncytial yolk has a nucleus been discovered equivalent to 

 the segmentation-nucleus formed from the fusion of the male and female pronuclei in the 

 germ. Dr Martin Barry, half a century ago (No. 21, p. 313), noted in the ovum of 

 Rana a nuclear body, which he described as elliptical, well defined in contour, apparently 

 granular, and placed within the membrana vitelli [vide Barry's figure, No. 21, pi. vi. fig. 

 28, d), but no such additional nucleus is apparently present in the Teleostean yolk. # The 

 emphatically passive and inert character of the Teleostean yolk has already been indicated, 

 and the real distinction of the active germ from its trophic appendage insisted on. 

 We have referred to the relation of the early blastomeres and the potential yolk-segments 

 Cunningham speaks of; but however plausible that view may appear during the first 

 stages of cleavage, it is difficult to maintain such a relation of blastomeres and yolk 

 when the morula is reached. The disc indeed becomes disengaged from the yolk (Gerbe 

 says it completely separates, No. 57, p. 330), and a series of independent phenomena begin 

 which concern it alone. We do not now allude to the formation of a true cavity beneath 

 the disc, as this phenomenon falls to be considered later, but to the embryological 

 separation between the germ and yolk, when their physical relations are most intimate. 

 Cunningham (No. 48), referring to the statement made by Agassiz and Whitman 

 (No. 2) that this separation dates from the 16-cell stage, observes, with greater accuracy 

 than the two authorities named, that this separation by a cavity is not seen in living 

 ova at the centre of the disc, and sections prove Cunningham to be right. In sections 

 the line of demarcation is broken by knob-like processes which project from the 

 blastoderm into the yolk (PI. II. fig. 1), and these appear to be masses of protoplasm 

 in the act of entering the disc, though another interpretation remains, viz. , that they are 

 pseudopodial protuberances.t During segregation and early segmentation remarkable 

 changes of form are seen in the Teleostean blastodisc — similar to the phenomena Schenk 

 noted in Elasmobranchs, and confirmed by Alex. Schultz (see Balfour, No. 10, p. 410), 

 consisting of an alternate rhythmical pullulation and subsequent flattening or subsidence — 

 a movement which involves the entire mass of the unsegmented disc (so that it seems 

 to draw together and become compact and prominent). This is shared by the individual 

 blastomeres in the segmented disc, as the separate cells appear at one time prominent 

 rounded bodies standing boldly out upon the yolk, at another time as conical or irregular 

 mounds (PI. X. figs. 9, 10), or again flattened structures, crescentiform in section, their 

 outline in the last case being less definite, and the entire disc exhibiting a diffuse and 



* See Balbiani, Comptes rendus, 1864, tome lviii. 



t Kowalewsky noted these transition-elements, and says that all stages can be seen amongst the entoblastic(yolk- 

 mass) cells forming below the blastoderm — from those which are still in the yolk to those which had entered the 

 blastodermic elements, and were only at one point of their bases united to the protoplasmic network of the yolk. 



