http://www.telusplanet.net/public/firstaid/ biology30/develop.htm

http://www.bartleby.com/ 107/illus39.html

http://www.visembryo.com/ baby/stage5.html

http://cats.med.uvm.edu/cats_teachingmod/embryology/1_3/week_2/movie_shells/circulation1.html

http://www.obgyn.net/us/gallery/ gallery.htm

http://www.bartleby.com/107/ illus502.html

http://user.gru.net/clawrence/ vccl/chpt1/fetcirc.HTM

http://www.bartleby.com/107/12.html