-
Fertilization
Fertilization is the fusion of haploid gametes (the sperm and egg) to form a diploid zygote. The male gamete is spermatozoon and the female gamete is oocyte. https://www.google.com/search?q=fertilization&safe=strict&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiO6cGv16fpAhX5j3IEHeWjDkcQ_AUoAXoECBQQAw&biw=691&bih=600#imgrc=oCTNhbhpe6qvYM -
Blastocyst
The blastula is the hollow cellular mass that forms in early development. Trophoblast make up the outer layer, the inner cell mass, and a blastocoele make up the blastocyst. The inner cell mass will eventually form the entire embryo.
https://www.google.com/search?q=blastocyst&safe=strict&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjR4cWM1afpAhUWg3IEHWlkDjQQ_AUoAXoECA4QAw&biw=691&bih=600#imgrc=D2E0PvPBT1yGwM -
Implantation
Implantation is the process in which the blastocyst attaches and invades the uterus endometrium. This process begins by the end of week 1 and extends through the second week of development. https://www.google.com/search?q=implantation&safe=strict&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiU6Ki716fpAhU1YjUKHYP9DzoQ_AUoAnoECBMQBA&biw=691&bih=600#imgrc=D9tOZ9hDCtGSLM -
Gastrulation
The epiblast layer, consisting of totipotential cells, derives all three embryo layers: ectoderm, mesoderm, and endoderm. The primitive streak is the region of cell migration from the epiblast layer, forming two germ cell layers becomes visible. https://www.google.com/search?q=gastrulation&safe=strict&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjqnvDx16fpAhVOmHIEHYtCC6wQ_AUoAXoECBAQAw&biw=691&bih=600#imgrc=8qf2bHuI6AaWRM -
Neurogenesis
Neurogenesis is the process by which new neurons are developed in the brain. When an embryo is first developing, this process is crucial. https://www.google.com/search?q=neurogenesis&safe=strict&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjB8b6-6afpAhUMmHIEHTyJDKoQ_AUoBHoECBUQBg&biw=691&bih=600#imgrc=2mKDO-AUw_F7vM -
Heart starts to beat
By day 23 of development, the heart begins to beat in humans. This become the first functioning embryonic organ formed. https://www.google.com/search?q=fetus+day+23&safe=strict&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiX47WZ6qfpAhValHIEHaa6A8oQ_AUoAXoECA0QAw&biw=691&bih=600#imgrc=WXp9i8dlo85BWM -
Neural tube
The neural tube is normally completely closed and the ventricular system is now separated from the amniotic fluid. The spinal cord ventral roots begin to develop. https://www.google.com/search?q=day+27+of+embryo+development&safe=strict&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjZkYfU6qfpAhX1lXIEHTb_A10Q_AUoAXoECA0QAw&biw=691&bih=600#imgrc=r3l_qj2INtvagM -
Placodes
Placodes refers to the process of ectoderm thickenings in the cranial region, which have important roles in the development of special sensory and other systems. In addition, there are integumentary places which are involved in hair follicle development. https://www.google.com/search?q=placodes&safe=strict&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiGx5PZ66fpAhUll3IEHcwJCXwQ_AUoAXoECA8QAw&biw=691&bih=600#imgrc=Ou2j5IdGj3SQOM -
stage 17
During stage 17, many new developments take place. The smell olfactory nerve fibers enter the brain, eyelid sulcus begin to deepen and eyelid folds develop, the pleuroperitoneal fold is no longer separated from the diaphragm, and the abdominal wall muscle cells have migrated 50% of the distance to the ventral midline. https://www.google.com/search?q=embryo+stage+17&safe=strict&source=lnms&tbm=isch&sa=X&ved=2ahUKEwj7x6qA7afpAhXloHIEHetmAeIQ_AUoA3oECA0QBQ&biw=691&bih=600#imgrc=CFxLuqb4ollGRM -
Stage 19
More important developments are taking place such as the arterial system, aortic arches, fusion of aortic and mitral endocardial cushion material, the first generation of sub segmental bronchi are now complete and the cloacal membrane ruptures. https://www.google.com/search?q=embryo+stage+19&safe=strict&source=lnms&tbm=isch&sa=X&ved=2ahUKEwijo8Pq7afpAhXog-AKHY7GA-IQ_AUoAnoECA0QBA&biw=691&bih=600#imgrc=mT3ULbhyyjI-UM -
Genitals
Mesenchyme interstitial cells secrete testosterone. Also, hCG stimulates testosterone production.
https://www.google.com/search? q=week+eight+embryo&safe=strict&source=lnms&tbm=isch&sa=X&ved=2ahUKEwj1_NbS7qfpAhU5knIEHRQ6A60Q_AUoAXoECA4QAw&biw=691&bih=600#imgrc=UItAKT8AhKKqAM