41 8 EMBRYOLOGY OP THE BRAIN AND SPINAL CORD. 



Stratum griseum centrale (in part) 

 Nuclei of third, fourth and part of fifth 



nerve 

 Ventral Zones -( XT , . , . , 



I Nuclei laterales supenores 



j Tegmenta (above isthmus) 



I Substantia nigra (above isthmus). 



Little is known about the development of the mid-brain. 

 The origin of the derivatives tabulated above is largely inferred 

 from their position and function. The formation of the corpora 

 quadrigemina has been observed (Figs. 120 and 124). In the 

 dorsal part of the mesencephalon, an elongated eminence on either 

 side of median line is present at the beginning of the third month. 

 Those two eminences resemble the corpora bigemina of birds, 

 fishes and reptiles. Two months later a transverse groove divides 

 each eminence into the superior colliculus and the inferior collic- 

 ulus. By the growth of fibers through the marginal velum be- 

 tween the colliculi and the corpora geniculata the brachium supe- 

 rius and the brachium injerius are produced (Fig. 43). The bases 

 pedunculi are produced, likewise, by the down-growth of fibers 

 from the fore-brain, which traverse the marginal velum in the 

 ventral region and, when medullated, form the prominent strands 

 of the adult. 



METENCEPHALON. 



The metencephalon is the fourth of the secondary brain vesicles. 

 From it are derived the isthmus, the cerebellum and the pons 

 (Figs. 17, 118, III, and IV, and 120). Its dorsal wall, which forms 

 the cerebellum, presents a transverse indentation, the metenceph- 

 alic angle ; and, as a result of that angle the opposite wall is bulged 

 forward. The ventral wall of the metencephalon forms the pons. 

 The cavity of this vesicle forms the upper half of the fourth 

 ventricle. Superiorly, this cavity contracts to the size of the cere- 

 bral aqueduct; it expands inferiorly and is broadest at the junc- 

 tion of the pons with the medulla oblongata. The ventricular 

 surface presents on each lateral wall a longitudinal furrow which 

 divides the metencephalon into a ventral and a dorsal zone. In 

 the adult the location of that furrow is indicated by the sulcus 



