Abstract
The human ovum arrives at the uterine cavity by day 4 of development. At that time the ovum is a morula or an early blastocyst. Hertig et al. (1954) have described a human morula of about 96 h developmental age found within the uterine cavity and consisting of 58 cells: 5 embryonic and 53 trophoblastic. All the trophoblastic cells were of a fairly uniform cuboidal nature. The zona pellucida was still present (Carnegie No. 8794). In Hertig’s paper is a description of a human blastocyst of 4½ days of developmental age and consisting of 107 cells: 8 embryonic and 99 trophoblastic. An embryonic disc was formed at this stage. The trophoblastic cells were thinned out, except those placed at the embryonic pole. These cells (the “polar trophoblasts”) make the first contact with the uterine epithelium at the implantation site.
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