74 THE EARLY DEVELOPMENT OF MAMMALS. 



upon their relations to the nerve-roots. In the ventral zone are lodged all the 

 neuroblasts which produce efferent nerve-fibers, which fibers constitute the ventral 

 nerve-roots in the region of the spinal cord (Fig. 39), and in the region of the 

 brain and the medulla oblongata produce both lateral and ventral roots. Neuro- 

 blasts are also differentiated in the dorsal zone, but the neuraxons which they send 

 out are confined in their distribution to the central nervous' system itself, and never 

 share in the formation of nerve-roots. The dorsal zone is further characterized by 

 the fact that the afferent or sensory fibers from the ganglia enter its lower edge 

 and have their first distribution within the dorsal zone. The stratification of the 

 thickened portions of the medullary tube begins early. It is initiated by the 

 appearance of a thin superficial layer, the ectoglia (Fig. fc2i, Ec.gl), which contains 

 no nuclei. The thick nucleated portion of the medullary wall changes in appear- 

 ance as the neuroblasts are differentiated. This occurs in such a way that there 

 is always a layer of relatively undifferentiated cells next the cavity of the tube. 

 This is known as the ependyma layer (Fig. 121, Eperi). It persists throughout life, 

 remaining thin, and never containing nerve-cells. The layer between the ependyma 

 and the ectoglia is called the gray layer, or cinerea (tin). This layer grows very 

 rapidly, and the enlargement of the spinal cord and brain depends chiefly upon the 

 expansion of the cinerea. 



The three cerebral vesicles pass through numerous modifications in their form 

 and cellular structure, yet their primary morphological characters are never obliter- 

 ated. The brain always has a central cavity, the boundary walls of which con- 

 stitute the organ. The fore-brain produces from its dorsal zone two lateral hollow 

 outgrowths, the cerebral hemispheres, the cavities of which constitute the lateral 

 ventricles of the adult. The median cavity of the fore-brain is the third ventricle; 

 the communication between the third ventricle and the lateral ventricle is the 

 foramen of Monro. The cavity of the mid-brain is always small, and its walls are 

 greatly thickened; it is termed the iter in the adult. The cavity of the hind-brain 

 is much enlarged and becomes the fourth ventricle. The part of the fore-brain 

 from which the hemispheres arise lies farthest cephalad, and is termed the telen- 

 cephalon. It is marked off on the dorsal side by a transverse fold, the velum 

 transversum, which projects inward. The part of the fore-brain caudad from the 

 velum is called the diencephalon. In the region of the fourth ventricle we can 

 observe that the cerebellum and the pons arise from the region adjoining the mid- 

 brain. This region is called the metencephalon, and the part caudad from it out 

 of which the medulla oblongata is differentiated is called the myelencephalon. The 

 narrow connection between the mid-brain and the hind-brain is termed the isthmus. 

 The following table indicates the relations of these embryonic divisions to the adult 

 parts. In Chapter VI the essential facts of brain development are illustrated and 

 described. 



