1178 EMBR YOL OGY. 



The first vesicle or prosencephalon sends out two hollow protrusions, which 

 spread rapidly, and in the walls of these nervous matter is developed, which con- 

 stitutes the cerebral hemispheres (Fig. 723, H), the cavities remaining as the 

 lateral ventricles. As these hemispheres extend they grow forward in front of 

 the anterior extremity of the primitive brain, and lie side by side, separated by the 

 longitudinal fissure ; they also grow upward, and again lying side by side are 

 separated by another portion of the same fissure, containing a thin layer of 

 mesoblast, which forms the falx cerebri ; behind and laterally they overlap the roof 

 and sides of the other cerebral vesicles, so that by the seventh month they project 

 behind them. In the floor of each of these hemispheres there occurs a local 

 thickening, which forms the corpus striatum, which is continuous behind with the 

 optic thalamus, presently to be described. The surface of the hemisphere is at 

 first smooth, but about the fifth month a sulcus or groove appears in either 

 hemisphere just external to the corpus striatum ; this is the fissure of Sylvius : 

 subsequently other fissures appear on the surface, three of which are of sufficient 

 depth to cause a projection into the lateral ventricle. These are the hippocampal 

 fissure, corresponding to the hippocampus major of the lateral ventricle; the 

 parieto-occipital fissure, corresponding with the bend of the posterior horn of the 

 ventricle; and the calcarine fissure, corresponding with the projection of the calcar 

 avis. 



The remainder of the first vesicle and the second, as we have seen, form 

 the third ventricle ; in its normal walls a thickening takes place, which forms the 

 optic thalamus. From the floor of this ventricle a hollow protrusion passes down- 

 ward, and is intimately connected with a diverticulum from the stomodseum, to 

 form the pituitary body or hypophysis cerebri (Figs. 720, 723, and 726). The 

 greater part of the roof of the third ventricle is very thin, and with the pia mater 

 forms the velum interpositum ; from its posterior part an outgrowth of cells forms 

 the pineal body or epiphysis cerebri. Where the cerebral hemispheres are not 

 separated in the middle line by the falx, in front and for some distance backward 

 over the roof of the third ventricle their mesial surfaces come in contact, and to a 

 certain extent fuse together, leaving however a small portion where no union 

 takes place, and thus a slit-like cavity is left ; this is termed the fifth ventricle, 

 though it will be at once seen that its development is quite different from that of 

 the other ventricles. Its lateral walls form the septum lucidum. The roof of 

 this cavity becomes thickened, and nerve-fibres pass across from the one hemisphere 

 to the other to form the corpus callosum, while in its floor longitudinal fibres are 

 developed to form thefomtx. 



The third vesicle, the cavity of which forms the iter a tertio ad quartum ven- 

 triculum, develops in its roof four well-marked thickenings, which together form 

 the corpora quadrigemina, while its lateral regions become thickened to form 

 the crura cerebri (Fig. 726). 



The dorsal surface of the fourth vesicle, or epencephalon, forms the covering 

 of the fourth ventricle, and in it a thickening occurs, which is developed into the 

 cerebellum ; its ventral and lateral regions form the pans (Fig. 726). 



In the fifth vesicle or metencephalon the lateral parts increase and grow down- 

 ward on each side toward the middle line, forming the medulla, while the dorsal 

 surface assists in forming the roof of the fourth ventricle. 



On making a transverse section of the lower part of the fourth ventricle, the 

 alar and basal laminae, already referred to as being present in the cord, are readily 

 recognized, while the thin roof-plate is seen to be greatly expanded laterally. 

 The dorsal part of the alar lamina becomes folded outward and downward, form- 

 ing what is termed the rhomboid lip (Fig. 727). This is at first separated by a 

 groove from the lateral aspect of the alar lamina, but ultimately fuses with it. As 

 the central canal of the cord opens out to form the fourth ventricle, the alar and 

 basal laminae come to occupy the floor of the ventricle the basal lamina lying 

 nearest the mesial plane. 



The Nerves. The nerves are developed, like the rest of the nervous system, 



