552 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [1'8] 



gjrow smaller as they approach the nucleus, arouud whicli they fonu a 

 <lis<'oidiil luass with tlie nuclear body imbedded in the center. The glo- 

 boids of this incipient germinal disk are very small but ovoidal in form 

 like the larger ones of the deutoplasm. They therefore simulate the 

 granules or niicrosomata of other protoplasm in general. The nucleus 

 still maintains its spherical form in the disk. It may be that in some 

 such way the protoplasm of the germinal disk of the eggs of osseous 

 fishes is developed. We find, in fact, that an oi)aque grauuhir stage 

 I)recedes the clear stage of maturity, by which time also the nucleus 

 has migrated from the center and disappeared in the rind of protoplasm 

 which now envelops the deutoplasm; this protoplasmic envelope now 

 constitutes the true hollow egg cell with the nucleus in a very eccentric 

 position imbedded in it or intimately blended with its substance, while 

 the deutoplasm may be looked upon as stored material or added ma- 

 terial, in short, rather as the cell contents than as an active part of the 

 cell itself. According to this view it also becomes of interest to note 

 that the i)osition of the oil spheres or drops in fish ova is due to the 

 same cause as that of the deutoplasm or food yelk. They are, in fact, 

 always more or less deeply imbedded in the deutoi)Iasni itself, and not 

 to any extent in the protoplasmic envelope. The fish-egg is, moreover, 

 directly comparable to a fat cell, in which large oil-drops have been 

 formed internally, the presence of which has compelled the nucleus to 

 assume a parietal position at some point in the superficial enveloping 

 protoplasm; just as in the fat cell the nucleus has been repelled from 

 its central position by the encroachment of the stored fat, the nucleus 

 of the Teleostean ovum has been displaced by the encroachment of the 

 stored deutoplasm. 



In the true first segmentation we also saw that the protoplasm was 

 differentiated into two parts, viz. a true germ disk capable of segment- 

 ing and developing the embryo, and a protoplasmic yelk envelope, the 

 function of which appeared to be entirely that of an approi)riative and 

 transformative membrane, histologically a syncytium, concerned mainly 

 in the elaboration of blood from the deutoplasm. The workings of the 

 general law which we have been tracing is evident wlierever we meet 

 with meroblastic or centrolecithal ova, and is, as we have seen, of great 

 physiological importance and invariably determines the plane of the 

 true first cleavage and consequently the relative dimensions of the germ 

 disk and yelk. It may be said, however, that the proportional bulk of 

 germ and yelk is probably determined by the higher laws of the strug- 

 gle for existence. It is also true that the bulk of the yelk has not the 

 slightest value in classilication, as great variations occur in resi)ect to 

 size within the limits of tlie same family of tishes. Again, it is singular 

 that the eggs of some amphibians should approach much more neaily 

 than those of the fishes the holoblastic or evenly segmenting type, 

 while the much smaller ova of many of the latter should i)resent us 

 with an extreme form of the meroblastic type. The only explanation 



