554 REPORT OF COMmSSIONER OF FISH AND FISHERIES. [100] 



iugested food -raateriais taken up voluntarily by the amoeba arc analo- 

 gous to the deutoplasm stored in the protoplasmic envelope of the mero- 

 blastic egg, and carried to it by the blood vascular network which 

 traverses the ovarian follicles ; with this difference again, that whereas 

 the transformation of ingested material by the amoeba is i)robably car- 

 ried on by the help of organic ferments developed during digestion, the 

 addition of new material to the growing egg is probably effected by a 

 cumulative process without the help of a ferment, the stored i)roteids 

 undergoing an actual retrogressive metamorphosis into non-contractile 

 globoids, or granular, globular, or ovoidal vitelline bodies in a condition 

 of stasis or quiescence. 



It also appears to be true in general that, whenever the layers of cells 

 comprising the whole or part of the yelk begin to segment, those of them 

 containing yelk material or granules have the nucleus more or less ex- 

 tensively displaced from its central position, which is in conformity with 

 the general principle stated. For convenience we may name those 

 forms of cells and ova which do not have the nucleus permanently and 

 notably displaced, as homogeneous or liomoplastic, and those which ex- 

 hibit marked permanent nuclear displacement as heterogeneous in com- 

 position, or as being heteroj)l(istic. 



This scheme does not exclude such types of cells as those of the uoto- 

 chord or the yelk-sack of a fish egg with its contained oil drops. We 

 find, indeed, that a large oil sphere may be the last part of the yelk to be 

 gradually broken down during yelk absorption, as in Gyhlum. The 

 reasons for regarding the yelk as a cell have already been stated, and 

 it will be needless to vindicate the claim of the chorda cells with their 

 large fluid cavities to that designation. The true first cleavage of the 

 Teleostean egg occurs when the germ disk is finally differentiated at 

 the time the first segmentation nucleus divides, leaving one half of the 

 latter in the jjlasmodium or yelk hypoblast, the other in the germ disk. 

 At this stage, therefore, the germ is a cell and the Plasmodium enveloi)e 

 covering the yelk another. The germ cell is the active animal cell; the 

 lower or yelk-containing cell is the passive and negative one, the con- 

 tents of which are mostly broken down during development by the 

 metabolic agency of the Plasmodium envelope. 



A set of phenomena are, however, to be considered in this connection 

 which must qualify the preceding general statements. I have been care- 

 ful to say that the nucleus of themeroblastic egg is permanently disphiced 

 when maturation is complete ; that is, even after the extrusion of the polar 

 cells. In holoblastic ova there is a marked recession of the remains 

 of the imcleus concerned in the first segmentation towards the center 

 of the egg after the extrusion of the polar cells ; in fact after being shoved 

 to the periphery to form the amphiasters and polar cells its remains 

 return to a more nearly central ])osition as the first segmentation nu- 

 cleus than that occupied by the germinative vesicle at the time the i^gg 

 was freed from the ovary. This is also a fundamental distinction be- 



