zygote 4-cell stage

4cell egg cell embryo development zygote vector pregnancy biology sperm ovule blastula anatomy gastrula blastomeres embryonic cage child birth cytoplasm division embryology endoderm female fertility fertilization fetus follicle genes gynecology health human infertility insemination medical medicine microscopic morula natural oocyte ovary ovulation penetrate reproduction reproductive seed stage fertilized treatment 4-cell stage development