DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 703 



(l) the blastodisc, or true segmenting mass ; (2) a granular layer, or subgerminal disc not 

 segmenting, and probably nutritive, and interposed between No. 1 and the vitellus. 



V. The Blastoderm. 



Within one or two hours after the entrance of the spermatozoa, the thickened cap 

 of protoplasm, either preformed as a discus proligerus, or segregated as a blastodisc 

 proper, undergoes segmentation. The blastodisc is readily distinguished with the 

 naked eye in the more transparent ova as a spot of lighter colour than the yolk 

 on which it is placed ; while under a lower power it is seen protruding as a discoid 

 prominence at either the upper or the lower pole, according to the particular form. 

 In certain Salmonidse, for instance, the germ always floats uppermost, as it also does in 

 the sterlet, according to Salensky, and in the trout ; this being due, according to Ransom, 

 to the oil attached to the disc, which compels it to float in the upper segment (No. 127, 

 p. 450). # In a number of pelagic ova, possibly in all, the disc lies underneath the yolk, 

 the animal pole being inferior ; but whether superior or inferior, the position is constant 

 for the species, and there is no actual reversal, such as occurs in Cephalopods, where the 

 germ and the yolk-pole exchange places at a certain stage. As the vitelline mass revolves 

 freely in the peri vitelline fluid, the germ may often be brought to the upper side by 

 agitation in the water ; but it usually seeks the lower pole at once, and remains there 

 when the egg is at rest. 



Balfour views the disc merely as a part of the ovum, which is characterised by the 

 presence of more protoplasm than the rest of the vitellus (No. 10, p. 106); but while 

 this is so in the Elasmobranch and Amphibian ovum, in the Teleostei the germ is so well 

 marked and distinct, and, with the exception of some colourless vesicles and a few granules, 

 so destitute of yolk-matter (apparently consisting of pure protoplasm) that the yolk 

 becomes rather an appendix than an essential part of the germ. 



The same author supposed that the Teleostean yolk at some later stage must be almost 

 entirely deprived of the protoplasm so abundantly interfused during the early stages, 

 and this undoubtedly is so, the yolk-matrix before it wholly disappears increasing in 

 density and coherency.t That the disc owes its origin to fecundation in all Teleosteans, 

 we have seen to be an error ; and the view of Coste, which Lereboullet adopts (No. 93, 

 p. 33), is not more tenable — viz., that the disc is derived solely from the divided and 

 scattered germinal vesicle — for, in some species, the discus proligerus is formed and this 

 vesicle is seated in its midst. As the segregation of the disc proceeds, and its mass 

 increases, its colour likewise becomes deeper ; and Ransom believes that it undergoes a 

 physical change, " being more solid " than in its earlier condition. 



The disc then is the essential part of the ovum, and the yolk is merely supplementary, 



* His figures the germ disc of Esox as uppermost (No. 67, Taf. i. fig. 13); but Lereboullet says, " Sa position est 

 oblique ou, si Ton vent inclined a l'equator " (No. 93, p. 481). 



t In a form like Anarrhichas the embryo remains long (several months) within the ovum, and when treated with 

 alcohol the yolk becomes extremely hard, and apparently consists of a purely albuminoid matrix. This likewise is the 

 case with the ovum of Salmo salar. Sea water also hardens the yolk of the latter species (vide No. 104a, p. 153). 



