The Carnegie collection contains no stage 1 specimens. The database for this stage is therefore limited to previously published images and videos of in vitro specimens that best show the progression of development. Only images of those specimens that the authors considered healthy were selected for inclusion in the database. Grateful appreciation is expressed to all of the authors and their publishers for their generosity in permitting the use of their images and videos in the database.
Stage 1 starts at the beginning of fertilization, when an oocyte is penetrated by a sperm, and ends with the intermingling of the paternal (male) and maternal (female) chromosomes at metaphase of the first mitotic division approximately 24 hours later. Stage 1 is divided into three substages; 1a, 1b and 1c. Stage 1a is referred to as the 'primordial embryo' and starts when the fertilizing sperm has passed through the zona (capsula) pellucida and its plasmalemma has fused with that of the oocyte. Penetration activates the embryo into resuming its arrested meiosis II and after anaphase it enters telophase with the expulsion of the second polar body. This marks the beginning of Stage 1b, often referred to as the 'pronuclear embryo'. The two pronuclei move toward each other and eventually lie adjacent near the center of the cell. The last phase of fertilization, and Stage 1, is Stage 1c, or the 'syngamic embryo'. The pronuclear envelopes disappear and the parental chromosomes come together in a process called syngamy. The embryo is now called a "zygote".
The diameter of the one cell (including the zona pellucida) is approximately 175 µm before fixation. The diameter of the cell, without the zona pellucida, is approximately 100 µm before fixation. As the embryo undergoes shrinkage with fixation and embedding and the cytoplasm is more affected than the zona pellucida, the subzonal (perivitelline) space becomes accentuated in fixed specimens.
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