RECENT FINDINGS IN THE INHIBITION OF AVIAN OOGENESIS 

Ralph G. Somes, dre 
1. Normal sequence of oogenesis. 
Oogenesis in the chick begins very shortly following 
fertilization. According to Romanoff (1960) the primordial 
germ cells can be recognized as early as 18 hours incubation, 
when they are still situated anterior and anterolateral to the 
embryo proper. This region is referred to as the “germinal 
crescent". According to Blocker, as stated by Romanoff, the 
germ cells become intermingled with the blood cells of the de~- 
veloping blood islands as the mesoderm moves forward between 
the ectoderm and the endoderm and surrounds the germ cells. 
At about 335 hours of incubation, the embryonic circulatory 
system ig continuous and the germ cells enter the embryo's: 
body and migrate, by 55 hours, to the gonadal regions. The 
germ cells distribute themselves very unequally between the two 
sides of the embryo with the left side receiving many more than 
the right. Between the eighth and eleventh days, the germ cells 
divide rapidly and decrease in size. They are then known as 
oogonia. During the 12th to 14th days, the cytoplasm increases 
and the enlarged oogonia become primary oocytes, Very little 
growth occurs between now and hatching, but between the 15th 
and 2list days, the nucleus goes through the first meiosis div- 
ision. Following hatching, follicle formation begins along 
with white yolk accumulation. The ova increase in size until 
they are about 4 mm, in diameter. At two to three months of 
age, the second meiosis division begins, but it is not completed 
until the ova is fertilized following ovulation. At 5 to 9 
@ays before ovulation, according to Marza and Marza, (Romanoff, 
1960), the ova'’s growth rate accelerates rapidly by the ad- 
dition of yellow yolk. By the time of ovulation, the ovum has 
usually attained a diameter of 35 mn. 
Ovulation itself is under hormonal control as is the 
yolk accumulation ee mentioned. Fraps and co-workers, as 
stated by Sturkie (1954), have demonstrated that ovulation is 
induced by the cyclic release of LH (luteinizing hormone) from 
the pituitary. This hormone is released by the pituitary 6 to 
8 hours prior to ovulation. As long as an egg is in the ovi- 
duct, further ovulation is inhibited, but upon oviposition 
another egg is usually ovulated within 15 to 75 minutes in the 
chicken and 4 to 5 hours in the pigeon according to Sturkie (1954). 
. Birds generally lay their eggs in cycles or clutches 
which can vary from one egg as with the auks and penguins to as 
many as 12 to 20 in partridges, (Romanoff and Romanoff, 1949). 
Removal of eggs from the nest may prolong egg laying in many 
wild birds and removal contributes to the year-round laying of 
the domestic hen. Booker and Sturkie (1950) have suggested that 
possibly thyroxine secretion rate may play some part in the role 
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