404 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



reagents the irregularity may be much less marked, or even entirely absent; and 

 this is true even of material taken from one and the same individual. 



Although the nuclear irregularity is thus seen to be dependent upon the 

 action of the fixing reagent, yet the striking temporal relation which this irregu- 

 larity shows to the condition of the cytoplasm, and thus to the egg's growth, 

 indicates that its occurrence is also dependent, in part at least, upon a physiological 

 condition of the egg peculiar to this period. The fact that the nuclear irregularity 

 is always well marked in material fixed in sublimate solution alone, while it is 

 much less marked, or even entirely absent, when the fixing reagent employed 

 contains a free acid, would seem at first sight to suggest that the nuclear irregu- 

 larity may be due to the slow action or penetration of the fixing reagent. This 

 suggestion, however, entirely fails to explain the cause of the direct temporal 

 relation between the nuclear irregularity and the stage of the egg's growth as 

 indicated by the condition of the cytoplasm. For not only is the egg. at the 

 stage marked by the greatest nuclear irregularity and cytoplasmic staining capacity, 

 invariably at the surface of the ovary, and therefore in a position to be most 

 quickly reached by the fixing reagent, but the younger eggs, in which the nucleus 

 is perfectly spherical, are usually in the interior of the ovary, either floating freely, 

 or tightly packed between the larger eggs. Moreover, eggs at this early stage are 

 occasionally found at the surface of the ovary, yet their nuclei are no less regularly 

 spherical. In the same way the form of the nucleus in adult, or nearly adult, eggs 

 shows no relation whatever to the position of the egg relative to the surface of 

 the ovary. The position of the egg in the pinnule, and thus also the facility offered 

 for rapid penetration by the fixing reagent, can not, therefore, be a factor in 

 determining the irregularity of the nucleus. 



The cytoplasm is the only cell constituent to the condition of which the nuclear 

 irregularity shows any definite relation, and here Professor Chubb believes is the 

 true cause of the nuclear irregularity seen in fixed material. 



In the younger oocytes the spherical form of the nucleus is determined by 

 the osmotic interchange taking place between the nutritive plasma of the ovary 

 on the one hand, and the caryolymph within the nucleus of the oocyte on the 

 other. As the egg grows there is a gradual accumulation of metoplastic material 

 in the cytoplasm, the presence of which must very greatly increase the osmotic 

 intensity at the cell wall while producing a corresponding decrease in the osmotic 

 intensity at the nuclear membrane. The nucleus, therefore, no longer possesses 

 the same powerful tendency to retain a spherical shape, and althougli it is possible, 

 indeed probable, that the actual deviations from this spherical form in life are 

 slight, they may obviously be greatly intensified by the action of the fixing reagent 

 in coagulating the cytoplasm. 



As yolk formation progresses the cytoplasm gradually loses its fluid character, 

 for although the yolk spherules are themselves as fluid as the original food-laden 

 cytoplasm, yet the conversion from a fine to a coarse alveolar structure is neces- 

 sarily accompanied by a firmer consistency of the alveolar mass as a whole. The 

 nucleus, therefore, is no longer free to assume a markedty irregular form during 

 fixation, or, to put it in another way, the fimier consistency of the coarsely alveolar 



