6th weeks
- The embryo measures 4 mm (1/8 inch) in length and begins to curve into a C shape.
- The heart bulges, further develops, and begins to beat in a regular rhythm. Septum primum appears.
- Branchial arches, grooves which will form structures of the face and neck, form.
- The neural tube closes.
- The ears begin to form as otic pits.
- Arm buds and a tail are visible.
- Pulmonary primordium, the first traits of the lung appear.
- Hepatic plate, the first traits of the liver appear.
- Buccopharyngeal membrane ruptures. This is the future mouth.
- Cystic diverticulum, which will become the gallbladder, and dorsal pancreatic bud, which will become the pancreas appear.
- Urorectal septum begins to form. Thus, the rectal and urinary passageways become separated.
- Anterior and posterior horns differentiate in the spinal cord
- Spleen appears.
- Ureteric buds appear.