126 BERTRAM G. SMITH 



The changes that immediately follow — the appearance of the 

 germinal vesicle at the surface, the rupture of its membrane, and 

 the reorganization of the germinal disc with the incorporation of 

 materials brought from the interior of the egg by the nucleus — 

 lead up to maturation and will be considered in the account of 

 that process. 



3. Resorption of ovocytes; the follicle cells in a phagocytic role. 



In young females nearing maturity (about 38 cm. body length), 

 a few ovocytes reach an advanced stage of development, becoming 

 filled with yolk and attaining a size nearly as great as the ovocytes 

 of an adult. These precocious ovocytes fail to undergo matura- 

 tion changes, and during the breeding season begin to degenerate, 

 or rather to be resorbed, together with some of the less advanced 

 ovocytes only partially filled with yolk. Viewed in the living 

 ovary, these degenerating ovocytes are colored a very bright yel- 

 low or orange. Digestion and absorption of the yolk granules 

 is accomplished through the medium of the cells of the follicular 

 layer proper, which become greatly enlarged and function as 

 phagocytes, thereby reversing their usual role as nurse cells to 

 the egg. 



The first step in the process of degeneration of the ovocyte 

 is the disappearance of the zona radiata; the later stages are 

 illustrated by figs. 27 to 30. The follicle cells enlarge, by increase 

 both in the size of the nucleus and in the amount of cytoplasm. 

 The zona pellucida is ruptured; at the same time it becomes irreg- 

 ularly thickened, a circumstance which may be interpreted either 

 as a shortening of the fragments due to the release of tension, 

 or as a step in thg process of dissolution. The rupture of the 

 zona pellucida allows the yolk to come in contact with the follicle 

 cells; the latter engulf the yolk particles, and become surrounded 

 by thin walls. About this time the zona pellucida disappears, 

 and the follicle cells are left as large yolk-filled cells resembling 

 columnar epithelium, forming a continuous layer around the egg. 



Digestion of the yolk particles is completed first at the outer 

 margin of the follicle cells, while the inner margin continues to 

 engulf yolk. The included yolk granules stain less deeply with 



