Within few hours after fertilization (while zygote is still in the fallopian tube) the zygote begins a series of mitotic divisions know as cleavage and forms 2, 4, 8, 16 daughter cells called blastomeres. The embryo with 8 to 16 blastomeres is called a morula, a solid ball of cells whose size is the same as that of the zygote. The cells of the morula continue to divide and transform into blastocyst as it moves further into the uterus.
Prior to implantation, the morula receives nutrition from a white, nutritive secretion called the uterine milk. It is composed of proteins and amino acids that provides nourishment to the developing embryo before getting implanted in the endometrium. The uterine milk is a part of the embryotroph and is secreted by the uterine glands. Embryotrophic nutrition is vital for early development and any kind of interruption may lead to malformations in the embryo.