THE OVUM 



125 



nucleoli are present, and they are sometimes of more than one kind, 

 as in tissue-cells.^ In many forms no centrosome or attraction-sphere 

 is found in the Qgg until the initial stages in the formation of the 

 polar bodies, though Mertens ('93) describes a centrosome and attrac- 

 tion-sphere in the young ovarian eggs of a number of vertebrates 

 (Fig. 79), while Platner ('89) and Stauffacher ('93) find what they 

 believe to be centrosomes in much later stages of Ajilostoim.tm and 

 Cyclas, lying outside the nuclear membrane. Beside these cases 

 should be placed those described by Balbiani, Munson, Nemec, and 

 others in which a body closely resembling an attraction-sphere is 

 identified as a "yolk-nucleus" or "vitelline body," as described at 

 page 158. In none of these cases is the identification of this 

 body wholly satisfactory, nor is it known to have any connection with 

 the polar mitoses. Most observers find no centrosome until the 

 prophases of the first polar mitosis. Its origin is still problematical, 

 some observers believing it to arise de tiovo in the cytoplasm (Mead), 

 others concluding that it is of nuclear origin (Mathews, Van der 

 Stricht, Ruckert), still others that it persists in the cytoplasm hidden 

 among the granules. In any case it is again lost to view after forma- 

 tion of the polar bodies, to be replaced by the cleavage-centrosomes 

 which arise in connection with the spermatozoon (p. 187). 



The egg-cytoplasm almost always contains a certain amount of 

 nutritive matter, the yolk or dentoplasm, in the form of liquid drops, 

 solid spheres or other bodies suspended in the meshwork and varying 

 greatly in different cases in respect to amount, distribution, form, and 

 chemical composition. 



I. The Nucleus 



The nucleus or germinal vesicle occupies at first a central or nearly 

 central position, though it shows in some cases a distinct eccentricity 

 even in its earliest stages. As the growth of the ^gg proceeds, the 

 eccentricity often becomes more marked, and the nucleus may thus 

 come to lie very near the periphery. In some cases, however, the 

 peripheral movement of the germinal vesicle occurs only a very short 

 time before the final stages of maturation, which may coincide with 

 the time of fertilization. Its form is typically that of a spherical sac, 

 surrounded by a very distinct membrane (Fig. 58); but during the 

 growth of the egg it may become irregular or even amoeboid (Fig. JJ), 

 and, as Korschelt has shown in the case of insect-eggs, may move 

 through the cytoplasm toward the source of food. Its structure is 



1 Hacker ('95, p. 249) has called attention to the fact that the nucleolus is as a rule 

 single in small eggs containing relatively little deutoplasm (ccelenterates, echinoderms, 

 many annelids, and some copepods), while it is multiple in large eggs heavily laden with 

 deutoplasm (lower vertebrates, insects, many Crustacea). 



