File:Week1 summary.jpg

Week 1 and 2 Human Development Overview

An overview of development of following fertilization during the first week and into the second week of development. The first week of development occurs floating freely within the uterine tube and body.

Note the timing is only approximate, it is the sequence of events that is important to understand.

• Day 14 of menstrual cycle - Oocyte released from ovary about the midpoint of the menstrual cycle.
• Day 0 - The haploid oocyte and spermatozoa fuse at fertilization to form a diploid zygote, the first cell. Fertilization occurring generally in the first third of the uterine tube (uterine horn, fallopian tube).
• Day 1 to 3 - Cell division by mitosis forming a solid ball of cells, the morula.
• Day 3 to 5 - Cell compaction forming a fluid-filled space and two cell populations, the blastocyst (1-chambered conceptus).
• Day 5 to 7 - blastocyst "hatches" from the surrounding zona pellucida.
• Day 7 to 9 - Implantation commences and continues through the second week of development.

Image Source: Stem Cells Information, NIH USA 2006 Image © Therese Winslow