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Carnegie Stage 3-2 Introduction

Stage 3 begins when a cavity first appears in the morula and ends after the zona (capsula) pellucida is shed and the embryo makes contact with the endometrial lining of the uterus. Stage 3 embryos are called free blastocysts. They have an estimated postfertilization age in vivo of between four and five days and measure 0.1 to 0.2 mm in diameter.

As there are only two stage 3 embryos in the Carnegie collection not all features of the stage can be seen in the serial sections. Therefore, we have supplemented the serial section databases with figures taken from descriptions of other stage 3 embryos. Because very few in vivo specimens have been described in the literature, most of the figures were obtained from in vitro specimens.


Stage 3 specimens have a postfertilization age of between four and five days in vivo. Embryo #8663 is one of two stage 3 embryos in the Carnegie collection. It is a 107-cell blastocyst estimated to be about 108 hours old.

Before fixation the specimen had no zona pellucida and measured 153 x 115 µm. After fixation the overall size of the embryo was 103 x 80µm. (Table of Dimensions; Hertig et al., 1954)

The specimen was collected and prepared for microscopic examination in 1949. It was fixed in 70% alcohol and Bouin’s fluid, embedded in celloidin paraffin, and serially sectioned at 6 µm. The sections were mounted on a glass slide and stained with H & E. Fifteen serial sections were obtained from the embryo, fourteen of them are included in the database. Structures are identified on every section image. The morphology of this embryo is well documented in the literature. It was first described by Hertig et al. in 1954.

Source: The Virtual Human Embryo.