292 



MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



and XXXVI their position and relations are well shown during the whole course of 

 embryonic development in the human brain. 



Having now pointed out the general relations of these groundwork or plan 

 fissures to the central hemisphere and manner of development in the foetus, it be- 

 comes necessary to inquire into their constancy throughout the Simiadae. I have 

 found that the evidence as derived from this source is not only in harmony with 

 the results to which I had been led by a study of the arrangement of the convo- 

 lutions in monkeys and Man, but it seems of itself sufficient to show that the 



Fig. 4. 



Fig. 5. 



0= occipital lobe; 0F= occipitofrontal lobe; OT= occipitotemporal lobe; S. S., Sylvian fissure; 

 t-rad., fissuratri-radiatus; M. a., mesial arched fissure; Cal., callosal fissure; Hp., hippocampal fissure; Ca., 

 calcarine fissure. 



fissures pointed out above as fundamental have really the importance and signifi- 

 cance ascribed to them. These fissures are present in all the monkeys as the best 

 marked fissures of the hemisphere, and we may trace, in a descending scale, the 

 gradual disappearance of the other fissures, until in the lowest form of monkeys, 

 the Arctopithecini or marmosets, they remain as the only fissures of the hemisphere, 

 as may be seen in the brain of Hapale jacchus and Midas, PI. XXXIV, figs. 11, 12 

 and 13. They are also the first to appear in the development of the foetal monkey 

 brain as shown in PI. XXXIV, figs. 27 and 28, which represent the condition as 

 found in a foetal Cefais apella. No other fissures are present in these brains, the 

 rest of the cerebral hemisphere remaining entirely smooth, resembling in this 

 respect the condition as found in the human foetal brain at the end of the sixth 

 month. 



Thus the brain of an adult monkey, Hapale midas, the brain of a foetal mon- 

 key, Cebus, and the brain of Man in one stage of his existence are similar as 

 regards type of fissuration: a beautiful illustration of that firmly established 

 principle that what is transient and embryonic in a higher form is permanently 

 represented in a lower ; and that the history of development of an organ tells 

 us the history of its origin. These facts of development and comparative anat- 

 omy appear to me to place almost beyond question the fundamental nature of 

 these fissures. I shall further attempt to show that the arrangement of the re- 

 maining fissures when they appear, points to the same conclusion. In Man and 



