538 H. F. Osborn 



are similarly disposed throughout its entire length. As in the earlier 

 stages the caUosal fibres ascend to the hippocampal sulcus and there 

 form two divisions. The anterior division cal extends horizontally 

 forwards as a compact bundle beneath the sulcus (fig. 26) distri- 

 buting its fibres around the fold into the mantle. The fibres of the 

 fornix, which is not a very large tract, unite with the posterior di- 

 vision cal' and pass backwards above the foramen of Monro and 

 downwards beneath the hippocampal sulcus (fig. 27). This tract cal' 

 fmb has precisely the same relations to the hippocampal fold both 

 in position and the distribution of its fibres as has been observed in 

 the anterior division of the corpus callosum. There can be no doubt, 

 from this observation, that the anterior and posterior divisions of the 

 embryonic corpus callosum are in like manner distributed to the 

 mantle, and that the latter which we recognize as the com mis- 

 sura cornu ammonis (Forel) is not a portion of the fornix 

 system, as has been generally held, but is primitively a portion of 

 the corpus callosum. The above evidence for this conclusion is sup- 

 ported by the study of the brain of Emys (figs. 18 and 15) and of 

 Columba also by the development of this commissure both in the 

 sheep and Kangaroo brain. This commissure, for which Forel's 

 designation is best retained, is unusually large in the marsupial brain, 

 a fact which undoubtedly led to Owen's error that the corpus cal- 

 losum proper was wanting in the marsupials. 



The development history of the marsupial brain thus gives us 

 three important results. First, it shows the typical distribution of 

 the anterior commissure. Second, it sliows the true relations of the 

 commissura cornu ammonis. Third, that the embryonic relations of 

 the corpus callosum and anterior commissure in the lower mam- 

 malia are similar to their adult relations in the Sauropsida (com- 

 pare figs. 16, 23, 24 and 25), thus completely establishing the homo- 

 logies of the cerebral commissures in the lower and higher verte- 

 brata which are advanced in this paper. 



General conclusions. 



In concluding the first part of this investigation, I left a gap in 

 the comparative history of the cerebral commissures between the 

 fishes (Teleosts), and Amphibia, which, the lack of necessary material 

 has prevented my filling. The fact that the embryonic Ceratodus 

 fore-brain resembles the embryonic amphibian fore-brain is sup- 



