Also the observations of Assheton on the primitive streak of the rabbit, and on the growth in length of the frog embryo, have led him to the conclusion that, as in the rabbit so in the frog, there is evidence to show that the embryo is derived from two definite centres of growth: the first, phylogenetically the oldest, being a protoplasmic activity, which gives rise to the anterior end of the embryo; the second, one which gives rise to the growth in length of the embryo. This secondary area of proliferation coincides with the area of the primitive streak, and he has shown, in a subsequent paper, by means of the insertion of sable hairs into the unincubated blastoderm of the chick, that a hair inserted into the centre of the blastoderm appears at the anterior end of the primitive streak, and subsequently is found at the level of the most anterior pair of somites. He then goes on to say-- "From these specimens it seems clear that all those parts in front of the first pair of mesoblastic somites--that is to say, the heart, the brain and medulla oblongata, the olfactory, optic, auditory organs and foregut--are developed from that portion of the unincubated blastoderm which lies anterior to the centre of the blastoderm, and that all the rest of the embryo is formed by the activity of the primitive streak area." In other words, the secondary area of growth, _i.e._ the primitive streak area, includes the whole of the spinal cord region, while the older primary centre of growth is coincident with the cranial region. In searching, then, for the origin of the segmental nerves, we must consider the type on which the cranial nerves are arranged rather than that of the spinal nerves. The first striking fact occurs at the spino-occipital region, where the spinal cord merges into the medulla oblongata, for here in the cervical region we find each spinal segment gives origin to three distinct roots, not two--a dorsal root, a ventral root, and a lateral root. This third root gives origin to the spinal accessory nerve, and in the region of the medulla oblongata these lateral roots merge directly into the roots of the vagus nerve; more anteriorly the same system {155}continues as the roots of the glossopharyngeal nerve, as the roots of the facial nerve, and as a portion, especially the motor portion, of the trigeminal nerve. Now, all these nerves belong to a well-defined system of nerves, as Charles Bell[1] pointed out in 1830, a system of nerves concerned with respiration and allied mechanisms, such as laughing, sneezing, mastication, deglutition, etc., nerves innervating a set of muscles of very different kind from the ordinary body-muscles concerned with locomotion and equilibration. Also the centres from which these motor nerves arise are well defined, and form cell-masses in the central nervous system, quite separate from those which give origin to somatic muscles.