1) Having no placenta 
• (a.) Belonging to the Aplacentata; without placenta.
Without a placenta; denoting the monotremes (which lay eggs and have no placenta) and the marsupials (which have a transitory simple yolk-sac placenta). ... (05 Mar 2000) ... 
(a-plә-sen´tәl) having no placenta.
Ap`la·cen'tal adjective [ Prefix a- + placental .] Belonging to the Aplacentata; without placenta. 
Type: Term Pronunciation: ā′pla-sen′tăl Definitions: 1. Without a placenta; denoting the monotremes (which lay eggs and have no placenta) and the marsupials (which have a transitory simple yolk-sac placenta).
Aplacental is a term applied to those mammals in which the young are destitute of a placenta. The aplacental mammals comprise the Monotremata and Marsupialia, the two lowest orders of mammals, including the duck-mole (ornithorhynchus), the porcupine, anteater, kangaroo, etc.
[adj] - having no placenta
aplacental Without a placenta.
adjective having no placenta; `monotremes and marsupials are aplacental mammals`
having or forming no placenta, as the marsupials and monotremes.