i found this on our local chicken chat grp; it lists what certain vitamin def. can cause. b12 is one that looks likely...the infant vitamins have it all:
Nutritional Deficiencies:
Depending on the degree of malnutrition, either death or physical symptoms will occur. Please refer to the below:
Nutrient
Deficiency Signs:
Vitamin A
Death at about 48 hours of incubation from failure to develop the circulatory system; abnormalities of kidneys, eyes and skeleton
Vitamin D
Death at about 18 or 19 days of incubation, with malpositions, soft bones, and with a defective upper beak prominent. Please
click on this link for info.
Vitamin E
Early death at about 84 to 96 hours of incubation, with hemorrhaging and circulatory failure (implicated with selenium).
Thiamin
High embryonic mortality during emergence but no obvious symptoms other than polyneuritis in those that survive.
Riboflavin (Vitamin B2)
Mortality peaks at 60 hours, 14 days, and 20 days of incubation, with peaks prominent early as deficiency becomes severe. Altered limb and beak development, dwarfism and clubbing of down are defects expressed by embryo.
Niacin
Embryo readily synthesizes sufficient niacin from tryptophan. Various bone and beak malformations occur when certain antagonists are administered during incubation.
Biotin
High death rate at 19 days to 21 days of incubation, parrot beak, chondrodystrophy, several skeletal deformities and webbing between the toes. Perosis.
Pantothenic acid
Deaths appear around 14 days of incubation, although marginal levels may delay problems until emergence. Variable subcutaneous hemorrhaging and edema; wirey down in poults.
Pyridoxine
Early embryonic mortality based on antivitamin use.
Folic acid
Mortality at about 20 days of incubation. The dead generally appear normal, but many have bent tibiotarsus (long leg bone), syndactyly (fused toes) and beak malformations. In poults, mortality at 26 days to 28 days of incubation with abnormalities of extremities and circulatory system.
Vitamin B12
Mortality at about 20 days of incubation, with atrophy of legs, edema, hemorrhaging, fatty organs, and head between thighs malposition.
Manganese
Deaths peak prior to emergence. Chondrodystrophy, dwarfism, long bone shortening, head malformations, edema, and abnormal feathering are prominent. Perosis.
Zinc
Deaths prior to emergence, and the appearance of rumplessness, depletion of vertebral column, eyes underdeveloped and limbs missing.
Iodine
Prolongation of hatching time, reduced thyroid size, and incomplete abdominal closure.
Iron
Low hematocrit; low blood hemoglobin; poor extra-embryonic circulation in candled eggs.
Source / Reference: http://gallus.tamu.edu/Extension publications/b6092.pdf