OKIGIN or THE CORPUS CALLOSUM. 49 



the true corpus callosum is wanting, to the Eutheria, in which alone that structare is 

 found. This relieves us from the necessity of discussing the writings of Oshorn and many 

 others who have hased their researches primarily ujion the examination of some sub- 

 mammalian form, and from this treacherous standj)oint have advanced liypotheses to 

 include the Mammalia. 



Since Flower (' Pliilosophical Transactions,' 1865) compared the cerebral commissures 

 of the jMarsupial with those of certain lowly-organized Eutheria, in ordej' to determme 

 the nature of the former, no one, so far as I know, has dealt Avith the subject from this 

 comparative standpoint. But while Elower compared the two series in order to throw 

 light upon the Marsupial, I am instituting the reverse process of comparing the 

 Cheiroptera with the more distinctly defined Metatherian tyj)e. 



Within the last three three years the labours of Blumenau *, Marchand f , and Paul 

 Martin + have added considerably to our knowledge of the process of development of the 

 corpus callosum in the higher mammals. And it is very instructive to compare the 

 2n"0gress of the developing commissvu-e in man or the cat with the stages of its jjhylogeny 

 with which this contribution deals. 



The following statements concerning the ontogenetic process are a free translation of 

 certain of Martin's conclusions {he. cit.) : — A thickening takes place in the lamina 

 lerminalis, which later disappears ; but before the disappearance the first fibres of the 

 corpus callosum apjiear in the dorsal j^art of the thickening. [Martin throughout uses 

 the term corpus callosum (Balken) in the sense of the whole dorsal commissure, 

 includhig the hippoemn'pal eomrnissnre. This will be apparent from the next statement.] 

 The first " callosal " fibres spring from the posterior columns of the fornix. The ventral 

 part of the corpus callosum [the psalterium] consists of fibres which run in the inner 

 Randbogen. The splcniimi is composed of fibres which extend from the inner and 

 outer Eandl)ogen to the other side. Tlie Ijody and genu of the corpus callosum consist 

 of fibres passing through the outer llandbogen. The cavuni septi pellucidi is formed 

 during the disajipearance of the thickened lamina terminalis, jmrtly in the place of the 

 disappearing thickening, but mainly by the inclusion of a part of the longitudinal 

 fissure of the cerebrum by the corpus callosum. For a long time it is open naso- 

 vcntralh^ Upon the ventral side of the corpus callosum a gradual " soldering " of the 

 hemisphere-walls takes place in the situation of the cavuni septi. The corpus callosum 

 increases by intussusception as well as by apposition of fibres. The striae lougitudinales 

 are formed in part by the upper llandbogen. The thickened lamina terminalis contributes 

 towards the formation of the caudal j)art of the striae mediales. 



In the subseqvient consideration of the results of our comparison we will see the 

 extent to which the ontogenetic process recapitulates the phylogenetic history. At the 



* '• Zur Eutwickelungsgeschichte iind feineren Anatomie des Hiriibalkens," Arcliiv fiir mikroskopisehe Anatomie 

 JBd. xxxvii. 1891, pp. 1-14, Taf. i. 



t " TJeher die Entwic-keluDg des Balkeus im menschlichen Gehirn," Arohiv fiir mikroskopische Anatomie 

 Bd. xxxvii. 1891, pp. 298-334, Taf. xv., xvi. 



X " Bogenfurche und Balkenentwickelimg bei der Katze," Jenaische Zeitschrift fiir Xaturwisieiisohaft, Bd. xxis. 

 1895, pp. 221-252, Taf. ii. and 13 figiu'cs in text. 



8* 



