We've featured photos from the Eye of Science before, but the website is so gosh darned awesome that we just had to do it again. This photo is of a cat embryo at 2 weeks gestation:
The brain (upper right) and the eye (black, upper centre) are visible, with the spinal cord running along the back to the tail (left side). The limbs have started to form (centre and lower right), and the remains of the umbilical cord can be seen at centre just above the tail (red). Internal organs can be seen under the front limbs (centre).
At this stage of development the heart and liver would be forming. A cat embryo takes 63-68 days to gestate. Magnification: x10.
http://www.eyeofscience.com/eos2/english/gallery/zoolog/frame.html (image 1) | The Eye of Science website