THE CEEEBETJM OK PBOSENCBPHALON. 179 



individual features, then it must spread out over this smaller radiate bundle 

 of fibers into folds. Such brain-folds, or Gyri, are lacking in only a few 

 mammals (lissencephalie mammals); in all others they are present in 

 greater or less abundance (gyrencephalic mammals). The arrangement of 

 the folds, conYolutions, or gyri, which is constant within certain limits for 

 single animals, depends, indeed, upon two factors: (1) upon the extension 

 of the brain-cortex which the species in particular has acquired in the course 

 of development, and (2) upon the relative size of the cranial cavity, .which 

 naturally must not proceed in equal steps with the cortical development, 

 since it is dependent upon other factors. 



One can, therefore, recognize no progressive development of brain- 

 fissuring within the animal series or even within a single family. For ex- 

 ample, in the Monotremata the Ornithorhynchus has a perfectly smooth 

 brain, while the Echidna has a richly convoluted one. Even among the 

 Primates the ape, Hapale, has a brain which is almost completely free from 

 convolutions. 



Not only on the richness of convolutions, but also on the course of the 

 convolutions, the two mentioned factors have an influence. When the brain- 

 surface of man is better known to you it Mali be advisable to cast a glance at 

 the various directions of the sulci in the animal series. 



It is the object here to show how, from modest beginnings, is developed 

 that great organ — the Mantle — which, as bearer of the highest psychic ac- 

 tivity, predominates over the lower brain-centers. 



The subject having now been developed to that point, the comparative 

 anatomical treatment of the subject should be brought to a close. 



If the foregoing presentation has been attentively followed, two points 

 will not have escaped the reader, namely: (1) that in different classes the 

 different train-segments may le developed in varying degree; (2) that there 

 are really lowly organized brains in which no single part has reached a high 

 development. Furthermore, the brain and spinal cord of the urodelate am- 

 phibians is, in the main, very little different from that of larval or embryonic 

 stages of higher animals. The medulla and spinal cord, especially, cor- 

 respond to those of the human embryo of about the second and third month. 

 In fact, the observation of tailed amphibians — the anura takes a somewhat 

 higher rank — teaches us that they lead a soulless dream-life and that they 

 are capable of psychic activities hardly recognizable to us now. 



In comparative psychological questions we still stand qui,te in the be- 

 ginning of our knowledge. That anatomical investigation can here usefully 

 co-operate — indeed, that to it it is granted to gain a certain insight just 

 where pure psychological observation is not yet sufficient — has possibly been 

 shown in these chapters in which the genesis of the brain has been followed. 



