“…Most therians (excluding haplorhine primates: Carter & Enders, 2016) exhibit a yolk sac placenta, and eutherians and the marsupial bandicoot form a chorioallantoic placenta (Freyer & Renfree, 2009; Hill, 1897; Mossman, 1987; Renfree, 2010). The diversity of mammal placentas has been studied extensively (see Mossman, 1987; Ramsey, 1975) and recent reviews offer valuable summaries of their structural and functional diversity (e.g., Carter, 2012, 2016, 2020; Carter & Enders, 2013, 2016; Carter & Mess, 2007; Enders & Carter, 2006, 2012; Ferner et al, 2014; Ferner & Mess, 2011; Smith, 2015). In the egg‐laying monotremes, the ovulated yolk is about 3–4 mm in diameter, and before oviposition, the egg swells with oviductal secretions to about 17 mm in length (Griffiths, 1978; Hughes & Carrick, 1978).…”