Fertilization AP Biology 2005 2006 Fertilization Joining of

Fertilization AP Biology 2005 -2006

Fertilization § Joining of sperm & egg u AP Biology sperm head enters egg 2005 -2006

Cleavage § Repeated mitotic divisions of zygote 1 st step to becoming multicellular u unequal divisions establishes body plan u § different cells receive different portions of egg cytoplasm & therefore different regulatory signals AP Biology 2005 -2006

Cleavage § zygote morula blastula u AP Biology establishes future development 2005 -2006

Organogenesis Mammalian embryo Umbilical blood vessels Chorion Bird embryo Amnion Yolk sac Allantois AP Biology Fetal blood vessels Placenta Maternal blood vessels 2005 -2006

Placenta § Materials exchange across membranes AP Biology 2005 -2006

Human fetal development 4 weeks AP Biology 7 weeks 2005 -2006

Sex determination Sperm Ovum Y Zygote XY X SRY Indifferent gonads No SRY X Ovum AP Biology X Sperm Develop in early embryo Testes XX Zygote Seminiferous tubules Leydig cells Ovaries (Follicles do not develop until third trimester) 2005 -2006

Human fetal development 10 weeks AP Biology 2005 -2006

Human fetal development 12 weeks AP Biology 20 weeks 2005 -2006

Human fetal development § The fetus just spends much of the 2 nd & 3 rd trimesters just growing …and doing various flip-turns & kicks inside amniotic fluid Week 20 AP Biology 2005 -2006

Human fetal development 24 weeks (6 months; 2 nd trimester) fetus is covered with fine, downy hair called lanugo. Its skin is protected by a waxy material called vernix AP Biology 2005 -2006

Human fetal development 30 weeks (7. 5 months) umbilical cord AP Biology 2005 -2006

Getting crowded in there!! 32 weeks (8 months) The fetus sleeps 90 -95% of the day & sometimes experiences REM sleep, an indication of dreaming AP Biology 2005 -2006