In the early embryo the formation of the line of cells below the primitive trace was described as the chorda dorsalis, the basis of the future backbone or vertebral column. Conversion of the gelatinous mass of cells into bone is the simple result of the deposition of bone-earth, calcium phosphate and carbonate mainly. With the ossification is associated the necessary elaboration of form of the bones, ending in the development of the bodies, arches, and processes of the vertebral bones, which are divisible into neck, back, loins, and tail - i.e. cervical, dorsal, lumbar, sacral, and coccygeal bones. At the anterior part of the vertebral column a prolongation of the structures occurs, to form the cranium to contain the brain, and next the bones of the face are formed from a series of arches derived from the visceral laminae, which have been described.

The four limbs or extremities at the same time are growing from the laminae which form the boundaries of the trunk, and it is noticeable that in all vertebrate animals the four extremities are at first identical in form, whether their ultimate use is to be for walking, grasping, swimming, or flying; in other words, whether the extremities are to be finally feet or hands, or fins or wings, they all have the same shape at first. The highest mammal in the course of embryonic development exhibits some of the features of the reptile, fish, and bird, a good example of evolution in a compressed form never exciting any astonishment, because it is never seen by the ordinary eye, being hidden in the membranes which invest the ovum, and only to be detected by elaborate and minute dissection by a practised anatomist skilled in the use of the most delicate instruments.