RT Book, Section A1 Chandrasoma, Parakrama A1 Taylor, Clive R. SR Print(0) ID 185152 T1 Chapter 15. Disorders of Development T2 Concise Pathology, 3e YR 1998 FD 1998 PB The McGraw-Hill Companies PP New York, NY SN 9780838514993 LK accessphysiotherapy.mhmedical.com/content.aspx?aid=185152 RD 2024/04/18 AB The initial divisions of the fertilized ovum (zygote) create a mass of cells (blastocyst) consisting of an inner cell mass (the embryonic disk) surrounded by an outer layer (the trophoblast). The cells of the trophoblast actively penetrate the endometrium and form the placenta.