The peripherally formed oil globules have become 

 larger and are moving toward the nuclear area. 

 Small yolk granules have appeared, some of which 

 have encroached upon the nuclear area, making 

 this area smaller than it was in the preceding 

 stage. These yolk granules stain mostly with 

 eosin, while the cytoplasmic ground substance 

 stains with both dyes. A homogeneous vitelline 

 membrane has made its appearance between the 

 oocyte and its surrounding follicle layer. 



Stage 6. — During this stage the follicle size is 

 between 0.45 mm. and 0.50 mm. The nuclear 

 membrane and chromatin material are no longer 

 evident; the oil globules have migrated around 

 the nuclear area and encroached upon it so that 

 the area now is relatively small. During this 

 migration, the yolk granules have moved away 

 from the central area, leaving this area surrounded 

 by oil globules. The small yolk granules of the 

 previous stage have now become very coarse. 

 The vitelline membrane under the single-celled 

 follicle layer is now thick and prominent. 



Up to this stage the oocyte appeared to be 

 isolecithal since the nucleus occupied a central 

 position as long as it was evident and the amount 

 of j T olk compared with the size of the nucleus did 

 not seem to be plentiful. Just prior to spawning, 

 however, the oil globules almost completely dis- 

 appear and the cell is so packed with large yolk 

 granules that no definite nuclear region is recog- 

 nizable. The tremendous amount of yolk, despite 

 the lack of information regarding the location of 

 the nucleus, suggests that the oocyte now is 

 telolecithal. 



Before the egg leaves the ovary, the single-celled 

 follicular layer ruptures and separates from the 

 vitelline membrane. The former remains in the 

 ovary where it gradually disintegrates. The 

 vitelline membrane becomes greatly distended by 

 imbibed water after the egg is spawned and 

 water-hardened. 



Most of the O-group gizzard shad already had 

 oocytes in Stages 1 and 2 as early as October. 

 After they appeared, they were a constant feature 

 in the ovary; they were most abundant in the fall 

 in O-group fish, somewhat less abundant in older 

 fish immediately after spawning, and least plentiful 

 in ripe fish. In the O-group shad the oocytes 

 begin development so late in the season that 

 usually they do not progress beyond Stage 2 by 

 late fall; there is an accumulation of Stages 1 and 



2. Older shad which spawned, however, showed 

 no such accumulation, for the oocytes that 

 formed earliest had time to progress to later stages. 

 Their constant presence — though they are not 

 plentiful in near-ripe and ripe fish — indicates 

 that one or both of the following processes must 

 occur: these early stages are continually being 

 formed, perhaps at varying rates; or the oocytes 

 formed later in the generative season — if there 

 be one — remained more or less dormant throughout 

 the remainder of the season — the nutrients being 

 diverted to the earlier formed, more advanced 

 oocytes. 



In shad which had spawned, oocytes in Stages 

 3 and 4 are found in late fall and winter, but in 

 the youngest shad (I group after January) they 

 appear in spring. 



Stages 5 and G usually do not appear in the 

 I-group fish; they are present in spring in the 

 II-group shad, and are found in spring and early 

 summer in the older females. 



SEASONAL CHANGES IN THE OVARY 



The seasonal changes in the ovary of every 

 gizzard shad are much the same after the first 

 spawning, which occurs usually in II-group fish. 

 Changes preliminary to first spawning follow a 

 different course. 



During fall, the oocytes of O-group fish appear 

 late and progress so slowly that by the time the 

 spawning season arrives (fish are now I-group) 

 the eggs are not mature enough to be spawned. 

 In a few fish of this age (presumably those which 

 hatched very early) some of the eggs become 

 mature and are spawned late in the spawning 

 season (fig. 10). In a few others, the eggs become 

 mature too late to be spawned and are resorbed 

 (fig. 12). In most of these I-group fish, however, 

 most of the eggs have developed to Stage 4 

 (fig. 11), and remain at that stage until the next 

 spawning season approaches. 



For convenience, the ovarian changes of gizzard 

 shad after their first spawning have been divided 

 into six stages (fig. 13). 



Stage A. — In late July, August, and early 

 September, the shrunken ovary is filled with 

 oocytes in Stages 1, 2, and 3. The germinal 

 epithelium extends inward from the periphery 

 of the ovary in irregular layers. The central 

 cavity of the saccular ovary is highly variable and 



GIZZARD SHAD IN WESTERN LAKE ERIE 



411 



