title: Morphologic stages of the equine embryo proper on days 17 to 40 after ovulation.
abstract: OBJECTIVE
  To describe the gross and histologic changes that develop in the equine embryo proper (ie, the portion of the embryo that becomes the fetus) from days 17 to 40 after ovulation and to compare the external features of equine embryos with those of porcine, ovine, and human embryos.
  
  
  METHODS
  34 embryos collected from mixed-breed pony mares.
  
  
  METHODS
  External features for each embryo proper, including length, number of branchial arches, growth of appendages, face and head features, and body features, were examined, using a dissecting microscope, for embryos collected on days 17 to 40. Internal features were histologically examined by serially sectioning embryos collected on days 20 to 35.
  
  
  RESULTS
  Number of embryos recovered for each day ranged from 1 to 5. The initial detection of features was not related closely to age; typically, the first attainment of a given body length or characteristic varied over a 3-day period among embryos. Similarly, the period during which individual characteristics for a given Carnegie stage were attained ranged from 3 to 6 days. Age at first appearance of a characteristic was greater for equine embryos than ages reported for ovine and porcine embryos but less than for human embryos. Indicators of age included number of pairs of branchial arches, all limb buds present, retinal pigmentation, and prominence of the pontine flexure.
  
  
  CONCLUSIONS
  No embryologic structures or changes were found that could be considered unique to equine embryos on days 17 to 40 after ovulation.
authors.affiliation: Department of Animal Health and Biomedical Sciences, School of Veterinary Medicine, University of Wisconsin, Madison 53706, USA.
authors.forename: D A
authors.initials: DA
authors.lastname: Acker
authors.forename: S
authors.initials: S
authors.lastname: Curran
authors.forename: E T
authors.initials: ET
authors.lastname: Bersu
authors.forename: O J
authors.initials: OJ
authors.lastname: Ginther
mesh_terms: D000818:Animals; D005314:Embryonic and Fetal Development / Q000502:physiology*; D005260:Female; D006736:Horses / Q000033:anatomy & histology / Q000196:embryology*; D006801:Humans; D011247:Pregnancy; D012756:Sheep; D013552:Swine