1905.] The Groivth of the Oocyte in Antedon. 



387 



structure to appear faintly stained. With the commencement of yolk 

 formation this avidity is greatly reduced and the products of chromatin 

 activity, now being produced in excess of the requirements of the cytoplasm, 

 accumulate on the chromatin threads, causing the increase in the staining 

 capacity of the latter and the formation of the basophile droplets. 



The Cytoplasm. — Throughout the whole growth period of the oocyte there 

 is a gradual accumulation in the cytoplasm of metaplastic material in 

 preparation for yolk formation. This accumulation is accompanied by an 

 increase in the basophile staining capacity, and fluid consistency, of the 

 cytoplasm. The process of yolk formation, that is the actual appearance of 

 the definitive yolk spherules, is unaccompanied by increased nuclear or 

 nucleolar activity, and consists simply in the rapid and automatic conversion 

 of the accumulated material into a form sufficiently stable to survive the 

 period of quiesence which succeeds the completion of the egg's growth. 



The " Germinal Vesicle." — Form of the Nucleus. — The oocyte constitutes an 

 osmotic system, of which the cell wall forms an outer, the nuclear membrane 

 an inner, semipermeable membrane. The accumulation in the cytoplasm of 

 soluble substances in preparation for yolk formation causes an increase in the 

 osmotic intensity at the outer membrane, and a corresponding decrease of that 

 at the inner. The loss of turgescence which thus results permits of the 

 assumption of an irregular form during fixation. 



Size of the nucleus and subdivision of the chromatin. — The large size of 

 the germinal vesicle, as also the fine subdivision of the chromatin, are due to 

 the limitations which the care of the specific qualities imposes on the 

 adaptive capacity of the chromatin ; both these conditions being merely the 

 expression of a more economic use of this material. The subdivision of the 

 chromatin increases the area of contact with the caryolymph, and the 

 increased area of the nuclear membrane through which diffusion takes place 

 permits of a greater concentration of nutritive material within the nucleus. 

 Up to the expansion due to the formation of the definitive yolk spherules, 

 therefore, the size of the germinal vesicle shows a close relation to that of 

 the egg, and is determined when equilibrium is established between the rate 

 of diffusion of nutritive substances through the nuclear membrane on the 

 one hand, and the requirements of the chromatin on the other. 



