CEREBRAL VENTRICLES IN THE PIG 233 



from comparing His's models of the interior of the human em- 

 bryonic brain with those of the pig is that they are strikingly 

 similar, especially in the earlier stages; but more careful study 

 shows very considerable differences. The 6.9-mm. human embryo 

 (His, '89, Taf. 1, Fig. 2) is somewhat more advanced than the 

 pig embryo of 5.1 mm., as indicated by the greater development 

 of the hemisphere, and of the pontine flexure. In His's model 

 the cavities of the parencephalon and synencephalon are not in- 

 dicated and the neuromeres of the hind-brain are not shown. It 

 may be questioned however, 'whether these structures are 

 actually absent from the human brain at this stage. On the 

 other hand, the model of the human brain shows a prominent 

 tegmental fold passing from the mid-brain toward the fore-brain, 

 but no such fold occurred in the pig embryo. In a model of 

 a pig embryo of 5 mm. Johnston ('09) has shown the neuromeres 

 of the fore-brain and hind-brain but there is no tegmental fold. 

 This fold is the portion of the tegmentum which projects dor- 

 sally into the cavity of the mid-brain; the fold on either side 

 extends from the median ventral sulcus to the sulcus limitans. 

 Altogether Johnston's model of the brain of the 5-mm. pig agrees 

 very closely with the first model described in the present paper, 

 and it is evident that we have more detailed and accurate knowl- 

 edge of the shape of the brain in the pig embryo than in the 

 human embryo of corresponding stage. 



His's model of the human embryo of 10.2 mm. corresponds 

 quite closely with the pig embryo of 12 mm. and his 13.6-mm. 

 specimen is comparable with the pig of 17 mm.; the human 

 embryo of the third month, which completes His's series of 

 models, is the stage intermediate between those dissected in pigs 

 of 22 and 45 mm. In all of His's models, except the oldest, the 

 tegmental fold is very prominent; and in the embryo of three 

 months it is perfectly distinct. The sulcus limitans is accord- 

 ingly well defined in the mid-brain, but it does not form a dis- 

 tinct continuous longitudinal groove from the spinal cord to the 

 optic recess in any of His's models. In fact. His described the 

 sulcus as becoming leveled off in the territory of the isthmus 



