The day finally dawned for Ueno Zoo to place its new star exhibit, the giant panda cub Xiang Xiang, on public display--and to avoid pandemonium, is keeping crowd numbers down.

Officials said that 2,000 or so lucky guests chosen by lottery gained admittance to the zoo in Tokyo's Taito Ward on Dec. 19 to view the female panda cub that turned 6 months old a week ago.

It is the first giant panda cub to go on show in the capital in 29 years, and huge crowds are expected in the coming months.

Zoo officials, taking stress levels of the mother and her cub into account, decided the lottery booking system would remain in place until the end of January and that only a maximum of 400 or so groups of five people or less would be allowed to view the pair behind a glass shield each day.

Xiang Xiang, a name that derives from the Chinese word for "fragrance," was born June 12, sparking a media frenzy after years of failed mating attempts.

Xiang Xiang is the third panda cub born at the zoo following Tong Tong in 1986 and You You in 1988.

Fujio Yamamoto, 57, who is in charge of rearing and exhibiting animals at the zoo, said that he and four other staff members worked around the clock to ensure no harm came to the mother and the cub, especially in the initial stages after the birth.

Their main concern was that the tiny cub would be crushed by its mother, Shin Shin, who weighs more than 100 kilograms, when she rolled over.

Yamamoto left another zoo to join Ueno Zoo and became the head of animal care in April 2015, and immediately revived the panda breeding project.

The key to successful panda mating is timing for the female. Zoo keepers need to have a potential mate on hand to place in the same enclosure as soon as the female displays signs of being ready.

Yamamoto and his panda team could not work out the signs at first.

After Shin Shin and Li Li, who went on to sire Xiang Xiang, failed to mate in 2016, Yamamoto dispatched the panda team to China to study panda-breeding techniques.

Thus it was that when spring came around the team did not miss the signs that Shin Shin was ready. After the dividing door that separates the male and female enclosures was opened, Shin Shin and Li Li immediately snuggled up to each other.

Once mating was confirmed, the team of panda watchers set up a grueling observation system to make sure nothing went wrong.

In June, Shin Shin's hormone level started to drop after showing a steady rise.

Around 3 a.m. on June 12, just a day after the team decided to monitor Shin Shin around the clock, Yamamoto received a call on the mobile phone he kept by his bed.

The panda team member reported that Shin Shin was “restless.”

Yamamoto rushed to the zoo in a taxi, and waited for Shin Shin to give birth in the special delivery room while tensely staring at a live-feed screen showing the interior of the delivery room.

Xiang Xiang arrived at 11:52 a.m. to the cheers of staff.

From then on, sleepless nights beckoned for the panda team. The members had to check the position of the tiny panda cub every half an hour to ensure it would not be crushed by its mother.

Observation logs were updated by the minute.

The 24/7 monitoring regime continued for over three months.

Despite their mental and physical exhaustion, the zoo keepers took comfort in the panda cub's steady weight gain and development.

“She is at the prime of her cuteness,” said Yamamoto, exuding a sense of adoration for the cub.

However, Ueno Zoo can only keep Xiang Xiang for a couple of years or so under an agreement with China concerning the joint breeding program.

She will be transferred to China at around the age of 2 to find a mate.

(This article was compiled from reports by Yuka Nishimoto, Kota Funahashi and Naomi Nishimura.)