The fetus inside the womb is actually called an embryo four weeks after it comes into existence. Around the time of implantation, certain cells within the blastocyst (an early stage of embryonic development) organize themselves into a group that will give rise to the body of the offspring. This marks the beginning of the embryonic period of development. The offspring is called an embryo until the end of the eighth week, after which it is a fetus. Eventually, the outer cells of the embryo togethermycontentbreak with cells of the maternal endometrium (wall of the uterus) form a complex vascular structure called the placenta. This organ serves to attach the embryo to the uterine wall, to exchange nutrients, gases, and wastes between the maternal blood and the embryonic blood, and to secrete hormones.
The embryo is shaped like a tadpole and is one-fifth of an inch long, about the size of a pea. Its heart has been beating for about a week. The head is defined, with eyes and rudiments of ears visible, and inside there is the beginning of a brain. So far, there are no bones. Tiny bumps are beginning to bulge where arms and legs will form. A week later, the embryo has grown to the size of a bean; the limb buds will have grown into little paws. The head has enlarged to accommodate the rapidly growing brain. The arms and hands become recognizable before the legs.
From the third month on, all sorts of finishing touches will be put on. Fingernails appear around the ninth week. Eyelids, formed at the beginning of the third month, seal the eyes shut like a newborn kitten’s until their development is completed. Facial features gradually shift: the eyes move closer together and the ear lobes move up where they belong from the earlier position low on the head. Lips form, cheeks fill out, taste buds develop on the tongue, and salivary glands come into being. The sex organs begin to produce the primitive eggs and male germ cells that are the seeds of the next generation - even as a woman is becoming a mother, her baby is preparing to make her a grandmother.