Over the course of roughly nine weeks, a fertilized canine oocyte develops into a newborn puppy. This incredible event in canine development is divided into two stages, embryological and fetal development, as the mass of cells divides and develops into organized and functional organs, tissue and bone. The embryological development of dogs -- which takes place in the first 30 days -- is responsible for laying out the basic structures of the body before the fetal stage, in which the organs, tissue and bone further develop and mature into one lovable, tiny, wiggling newborn puppy at birth.
In the first week of embryonic development, sperm penetrates the egg, and a chemical reaction causes the egg's shell surface to bind, preventing additional sperm from implanting. As soon as this fertilization occurs, genetic programming will take over the new two-celled mass, and the first cellular division -- or dividing of the cells -- occurs. Every 12 hours, the cell mass will divide as it journeys toward the uterine horn, the point where the uterus and the fallopian tubes meet. The first eight cells created are undifferentiated, meaning they each have the potential to become any cell type necessary in the developing embryo. In a publication titled "Fetal Development & Birth Defects in Dogs," written by Veterinarian Bretaigne Jones, "only three of these [eight] cells are necessary to grow an entire embryo, which will continue to develop through the fetal stage, into a puppy."
At the start of the second week, the cell mass -- which will have divided itself into a 16-cell mass -- will continue dividing as it arrives at the uterine horn and enters the uterus. Once inside the uterus, the cell mass is known as an active embryo. The embryo will continue to rapidly develop. By the end of the second week, the small embryo will have divided itself into a 64-cell mass. Also within two weeks, the embryo will find a place of its own in the uterus and bury itself into the tissue of the uterine wall. Then, the placenta -- an organ essential to providing the embryo with nourishment and oxygen -- will form.
Weeks 3 and 4
In the beginning of the third week, the development of the basic structures of the puppy embryo begins. The primary structures of the eyes, spine, legs and paws will begin to form, as well as the beginnings of organs. During the fourth week, the embryo will grow from 5 to 10 millimeters in length to 14 to 15 millimeters. During these last weeks of the first month, the embryo is most susceptible to defects. According to Veterinarian Bretaigne Jones in the article "Fetal Development & Birth Defects in Dogs," the embryo stage lasts until the end of this fourth week and "is marked by the differentiation of the early cells into general cell types, which will further specialize mid-gestation."
Weeks 5 Through 9
In the fifth week, the embryonic development stage transitions into the fetal stage, and the development of features such as toes, whisker buds and claws will form. The fetus begins to look more like a puppy every day. A veterinarian will be able to determine the gender and hear a fetal heartbeat at this stage. In the final few weeks of development, Weeks 8 and 9, the canine fetus will finish development of the majority of organs, bones and tissue and continue to mature and gain weight. While development is not complete, puppies can be born safely at the eighth week. Because dogs typically produce litters of multiple puppies, the uterus will become crowded as the puppies continue to grow and prepare for the journey through the birth canal during birth.