Fig.175. Human embryo, five weeks old, half an inch long, seen from the right. Fig. 175—Human embryo, five weeks old, half an inch long, seen from the right, magnified. (From _Russel Bardeen_ and _Harmon Lewis._) In the undissected head we see the eye, mouth, and ear. In the trunk the skin and part of the muscles have been removed, so that the cartilaginous vertebral column is free; the dorsal root of a spinal nerve goes out from each vertebra (towards the skin of the back). In the middle of the lower half of the figure part of the ribs and intercostal muscles are visible. The skin and muscles have also been removed from the right limbs; the internal rudiments of the five fingers of the hand, and five toes of the foot, are clearly seen within the fin-shaped plate, and also the strong network of nerves that goes from the spinal cord to the extremities. The tail projects under the foot, and to the right of it is the first part of the umbilical cord.
How the five fingers or toes with their blood-vessels gradually differentiate within the simple fin-like structure of the limbs can be seen in the instance of the lizard in Fig. 174. They are formed in just the same way in man: in the human embryo of five weeks the five fingers can clearly be distinguished within the fin-plate (Fig. 175).
The careful study and comparison of human embryos with those of other vertebrates at this stage of development is very instructive, and reveals more mysteries to the impartial student than all the religions in the world put together. For instance, if we compare attentively the three successive stages of development that are represented, in twenty different amniotes we find a remarkable likeness. When we see that as a fact twenty different amniotes of such divergent characters develop from the same embryonic form, we can easily understand that they may all descend from a common ancestor.
Figs. 176-178. Embryos of the bat (Vespertilio murinus) at three different stages. Figs. 176–178—Embryos of the bat (_Vespertilio murinus_) at three different stages. (From _Oscar Schultze._) Fig. 176: Rudimentary limbs (_v_ fore-leg, _ h_ hind-leg). _l_ lenticular depression, _r_ olfactory pit, _ok_ upper jaw, _uk_ lower jaw, _ k_2, _k_3, _k_4 first, second and third gill-arches, _a_ amnion, _n_ umbilical vessel, _d_ yelk-sac. Fig. 177: Rudiment of flying membrane, membranous fold between fore and hind leg. _n_ umbilical vessel, _o_ ear-opening, _f_ flying membrane. Fig. 178: The flying membrane developed and stretched across the fingers of the hands, which cover the face.