DEVELOPMENT OF THE BRAIN.
The fundamental form is the simple brain-tube. In consequence of increased growth in certain parts and diminished growth in others, the brain-tube early exhibits a segmentation. At first it consists of three dilatations, the primary separated by two annu lar constrictions, the vesicles being desig nated as anterior, mid dle and posterior. From these three brain-vesi cles later arise the three chief divisions : the Fore-brain or Prosen cephalon, the Mid-brain or Mesencephalon and the Hind-brain or Rhombencephalon. The three primary vesicles subsequently give rise to five secondary since the fore-brain differen tiates into the telencephalon and the diencephalon, while the hind-brain divides into the metencephalon and the myelen cephalon. The hind-brain is sepa rated from the mid-brain by a narrow constricted segment, the isthmus (isthmus rhombencephafi). The mye lencephalon is continuous with the spinal cord. The primitive tube, therefore, differentiates into six divisions (Fig. 3) : the telencephalon, the diencephalon, the mesencephalon, the isthmus, the metencephalon and the myelencephalon.
In the later stages, the devel opment of the nervous substance is especially vigorous in the two lateral walls of the neural tube, while the median are as of its floor and roof (the floor- and roof-plates) for the most part remain thin and epithelioid. The different divisions of the brain-tube participate in the further development in very unlike degree. Certain seginents remain far behind, while others far outstrip their roundings in consequence of their vigorous growth. Along with the displacement of certain train-segments induced by unequal growth, other processes contribute to the ment of the original fundamental plan of the whole. Among such factors belong partic
ularly the appearance of robust cross-fibres (corpus callosum, pons). Consequently it is impossible to mark off superficially the individual segments on the brain of the adult.
The developmental relations of the parts of the brain to the individual brain-vesicles is best explained by the accompanying table after His. It will serve as guide in the con sideration of the morphology.
The prosencephalon and the mesencephalon together are also designated the cere brum or great brain. The brain-stem (truncus cerebri) embraces the so-called the brain ganglia; it consists of the stem of the end-brain, the inter-brain, the mid-brain, the isthmus, the and the medulla oblongata.
The cavities of the embryonal brain-vesicles likewise change their form under the influence of the various growth-processes. The central canal of the spinal cord is continued into the hind-part of the myelencephalon. The cavity of the fore - part of the myelencephalon and that of the entire metencephalon become the fourth ventricle, The cavity of the mid-brain remains as the aquaeductus cerebri or Sylvian aqueduct. The cavity of the dien cephalon or inter - brain becomes the third ventricle, which communicates with the lateral ventricles — the cav ities of the hemisphere - vesicles — by means of the Y-like foramen of Monro (foramen interventriculare). All these spaces are filled with a fluid, the liquor cerebro-spinalis.