410 EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



sists, in the calcar avis. The primary occipito-parietal fissure 

 entirely disappears. The whole superior part of the hippocam- 

 pal fissure is represented in the adult, as to position, by the cal- 

 losal fissure. The lateral cerebral fossa (Fig. 122) is a deep bay 

 in the ventral border of the hemisphere vesicle. It is due to the 

 relatively limited growth of the corpus striatum in comparison 

 with the more rapid and greater growth of the surrounding parts. 

 The thickened floor of the fossa cerebri lateralis develops, inter- 

 nally, the corpus striatum and, externally, forms the insula (Reili). 

 The lateral fossa and the four fissures mentioned above are well 

 formed by the third month, but the lateral cerebral fossa is not 

 converted into a fissure until the end of the fifth month. Even 

 then only the posterior ramus of the fissura cerebri lateralis (Sylvii) 

 is formed, and this is brought about by the meeting of the temporal 

 and the fronto-parietal parts of the operculum. It is during the 

 first year after birth that the development of the orbital and 

 frontal parts of the operculum produces the anterior horizontal 

 ramus and the anterior vertical ramus of this fissure. 



Secondary Sulci and Secondary Fissure (Fig. 123). The 

 secondary sulci are linear indentations of the surface only; they 

 cause no ventricular eminences. About the middle of the fifth 

 month the first secondary sulcus makes its appearance. It is 

 the sulcus cinguli of the medial surface, which separates the su- 

 perior frontal gyrus and paracentral lobule from the gyrus cinguli 

 of the adult brain (Fig. 27). Its development in two or three 

 pieces explains its irregularity and occasional want of continuity. 

 In the sixth month the subparietal and occipito-parietal sulci are 

 present. There is no sign of the latter sulcus in the early part 

 of the fifth month and the permanent sulcus does not produce a 

 ventricular eminence, hence the occipito-parietal indentation is a 

 sulcus and not a fissure (see Cunningham Memoirs). Early in 

 the sixth month the most important of the remaining sulci are 

 formed, such as, the central sulcus (Rolandi), the precentral, the 

 postcentral, the superior temporal sulcus and the remaining larger 

 sulci of the frontal, parietal, occipital and temporal lobes (Figs. 

 23 and 123). The only secondary fissure, the collateral fissure 

 appears in the same month. It is developed on the tentorial sur- 



