The metamerism of the hyposoma is less conspicuous because in all the craniotes the cavities of the ventral segments, in the walls of which the sexual products are developed, have long since coalesced, and formed a single large body-cavity, owing to the disappearance of the partition. This cenogenetic process is so old that the cavity seems to be unsegmented from the first in all the craniotes, and the rudiment of the gonads also is almost always unsegmented. It is the more interesting to learn that, according to the important discovery of Ruckert, this sexual structure is at first segmental even in the actual selachii, and the several gonotomes only blend into a simple sexual gland on either side secondarily. (FIGURE 1.171. Human embryo of the fourth week (twenty-six days old), one-fourth of an inch in length magnified twenty times, showing: point of development of the hind-leg, umbilical cord (underneath it the tail, bent upwards), trigeminal nerve V Trigeminus, optic-muscle nerve III Oculo-motorius, rolling muscle nerve IV Trochlearis, rudiment of ear (labyrinthic vesicles), pneumogastric nerve X Vagus, terminal nerve XI Accessorius, hypoglossal nerve XII Hypoglossus, first spinal nerve, point of development of arm (or fore-leg), true spinal nerve. (From Moll.) The rudiments of the cerebral nerves and the roots of the spinal nerves are especially marked. Underneath the four gill-arches (left side) is the heart (with auricle, V and ventricle, K), under this again the liver (L).) Amphioxus, the sole surviving representative of the acrania, once more yields us most interesting information; in this case the sexual glands remain segmented throughout life. The sexually mature lancelet has, on the right and left of the gut, a series of metamerous sacs, which are filled with ova in the female and sperm in the male. These segmental gonads are originally nothing else than the real gonotomes, separate body-cavities, formed from the hyposomites of the trunk.