Question: When did chicken pho first appear?
About the article "Pho", section "Development".

Candidate passages:
Pho originated in the early 20th century in northern Vietnam, apparently southeast of Hanoi in Nam Định Province, then a substantial textile market.
The traditional home of pho is reputed to be the villages of and (or ) in Đông Xuân commune, Nam Trực District, Nam Định Province.
According to villagers, pho was eaten in long before the French colonial period when it was popularized.
Pho was originally sold at dawn and dusk by roaming street vendors, who shouldered mobile kitchens on carrying poles (').
From the pole hung two wooden cabinets, one housing a cauldron over a wood fire, the other storing noodles, spices, cookware, and space to prepare a bowl of pho.
Pho vendors kept their heads warm with distinctive, disheveled felt hats called '.
Hanoi's first two fixed pho stands were a Vietnamese-owned Cát Tường on Cầu Gỗ Street and a Chinese-owned stand in front of Bờ Hồ tram stop.
They were joined in 1918 by two more on Quạt Row and Đồng Row.
Around 1925, a Vân Cù villager named Vạn opened the first "Nam Định style" pho stand in Hanoi. ""
declined in number around 1936–1946 in favor of stationary eateries. 
The several regional variants of pho in Vietnam, particularly divided between "northern pho" () and "southern pho" or "Saigon pho" ("").
Northern pho tends to use somewhat wider noodles and much more green onion, and garnishes offered generally include only vinegar, fish sauce and chili sauce.
On the other hand, southern Vietnamese pho broth is slightly sweeter and has bean sprouts and a greater variety of fresh herbs.
The variations in meat, broth, and additional garnishes such as lime, bean sprouts, "ngò gai" ("Eryngium foetidum"), "húng quế" (Thai/Asian basil), and "tương đen" (bean sauce/hoisin sauce), "tương ớt" (hot chili garlic sauce, e.g., Rooster Sauce) appear to be innovations made by or introduced to the South.
Southerners eat pho for breakfast and occasionally lunch, whereas northerners eat pho at any time of day.
International variants include pho made using tofu and vegetable broth for vegetarians (""), and a larger variety of vegetables, such as carrots and broccoli.
Many pho restaurants in the United States offer oversized helpings with names such as "train pho" ('), "airplane pho" ('), or "California pho" (').
In some parts of the United States, fresh ' is not widely available.
Dried noodles called ' are often used instead.
Some restaurants may serve ' (fresh pho noodles) upon request.
Pho has been adopted by other Southeast Asian cuisines, including Hmong cuisine. 
Pho is served in a bowl with a specific cut of white rice noodles in clear beef broth, with slim cuts of beef (steak, fatty flank, lean flank, brisket).
Variations feature tendon, tripe, or meatballs in southern Vietnam.
Chicken pho is made using the same spices as beef, but the broth is made using only chicken bones and meat, as well as some internal organs of the chicken, such as the heart, the undeveloped eggs and the gizzard. 
"Dir", directed, "pho", photographed; "pro", produced.
"The Artist's Horse".
20 minutes, for "The South Bank Show", 1978.
"Dir, pho, pro" "Beehives and Runaway Wives".
For the Discovery Channel, 2002.
"Dir, pho" "Bull Magic".
For "Under the Sun" (BBC) and National Geographic, 1994.
"Dir, pho, pro" "Dances with Llamas".
50 minutes, for "Under the Sun" (BBC), 1997.
"Dir, pho, pro" "Empty Quarter".
50 minutes, for "Journeys" (BBC), 1996.
"Dir, pho" "Fat Fiancees".
For the Discovery Channel, 2005.
"Dir, pho" "Finite Oceans".
1995.
"House of the Spirits".
For the Discovery Channel.
"Dir, pho" "How Does It Feel?".
Pictures that Move, 1976.
"Pho" "Månen är en grön ost".
72 minutes, Stiftelsen Svenska Filminstitutet, 1977.
"Pho" "Mud and Water Man".
For the BBC, 1973.
"Pho" "A Mysterious Death".
49 minutes, for the BBC, 1999.
"Dir, pho" "Now Is the Hour".
1970.
"Dir" "The Painter and the Fighter".
For "Survival" (Anglia), 1996.
"Dir, pho" "Queen of the Elephants".
90 minutes, for the Discovery Channel, 1994.
"Pho" "The Search for Shangri-La".
50 minutes, for the BBC and PBS.
"Dir, pho" "Stick Fights and Lip Plates".
50 minutes, for the Discovery Channel.
"Dir, pho" "Sunday Pursuit" (or "Love at First Sight").
25 minutes, 1990.
"Pho" "The Tide of War."
50 minutes, for National Geographic, 1991.
"Pho" "Up North".
1970.
"Dir" "Vincent the Dutchman".
50/52/60 minutes, for "Omnibus", 1972.
(Winner of a BAFTA award for "Television: Specialised Programme" in 1973.)
"The Witchdoctor's New Bride".
50 minutes, for the Discovery Channel, 2005.
"Dir, pho" "Women of the Yellow Earth".
50 minutes, 1994.
"Dir, pho"  
Pho (pronounced variously as , , , or ; from , pronounced ) is a Vietnamese noodle soup consisting of broth, linguine-shaped rice noodles called "", a few herbs, and meat.
Pho is a popular street food in Vietnam and the specialty of a number of restaurant chains around the world.
It is primarily served with either beef or chicken.
The Hanoi and Saigon styles of pho differ by noodle width, sweetness of broth, and choice of herbs.
The origin of pho and its name is a subject of scholarly debate.
A related beef noodle soup, "bún bò Huế", is associated with Huế in central Vietnam.