In all Craniates the dorsally-placed and tubular central nervous system has its anterior portion enlarged and otherwise modified to form a "brain," while the remaining portion, retaining a simpler and more uniform structure, forms the spinal cord. In the embryo the brain always consists of three successive sac-like enlargements known as the fore-, mid-, and hind-brain, and from {144}these are developed the various parts of the complex adult brain, which in the disposition and mutual relations of its parts conforms to a common plan in all the members of the group. There are at least ten pairs of cranial nerves having their origin from the brain, and, in addition, a varying number of spinal nerves arising from the spinal cord, and as a rule formed in each case by the union of a mainly sensory, ganglionated, dorsal root with a mainly motor, non-ganglionated, ventral root. The median and usually vestigial, parietal, or pineal eye may sometimes be retained as a functional organ, but there exist in all Craniates, in addition, paired eyes, the sensory portion of which, the retina, is derived as an outgrowth from the first of the primary embryonic brain-vesicles. To these organs of special sense are added a pair of auditory organs, and a pair of olfactory organs, besides, in the lower aquatic Craniates, the peculiar sensory organs of the "lateral line." The gonads are reduced to a single pair in the adult, although it is possible that they may have a multiple origin in the embryo. Gonoducts for the discharge of the sex-cells are almost invariably present, and may owe their origin either to a change of function on the part of certain kidney-ducts, or to independent evolution from the lining membrane of the coelom. The ova are generally provided with a large amount of nutritive reserve in the shape of food-yolk, and hence the process of segmentation is frequently partial or "meroblastic," but in some groups, in which the ova have less food-yolk, it is complete or "holoblastic." The typical invaginate gastrula stage, which is so striking a feature in the embryonic history of the lower Chordata, occurs also in a few of the lower Craniates, but in most of them it is apt to become masked or modified in various ways by the presence of a superabundant amount of food-yolk. Functional hermaphroditism is of very rare occurrence in Craniates, and, as in the Cephalochordata, reproduction by budding and the formation of colonies are unknown. Thus distinguished from other Chordata, the Craniata are divided into six "classes," which may be variously grouped, as the following table shows:—