COMMENTARY ON CHAPTER X. 400c 



lias been observed in the lamprey and in Amphibia (Osborn, 2, 268), and the name has 

 been adopted by the writer last named (2, 268, 4). 



(D) The general constitution of the brain. In the first edition (pp. 404, 409, 410, 

 413), in accordance with the commonly accepted view based upon the more obvious condi- 

 tion of the parts in the adults of the higher vertebrates, the entire brain was regarded 

 as comprising two principal regions, a caudal, unpaired and a cephalic, paired. After 

 careful reconsideration of the matter, especially upon the embryological and other 

 grounds presented by the senior author (oC, 145-146), we are led to regard the brain as 

 consisting essentially of a single series of segments and cavities, although some of them 

 may present lateral protrusions, so that the original, simple cavity becomes triple, with 

 one mesal, and two lateral portions. 



Leaving out of view the suggestion that the embryonic optic vesicles may represent 

 the lateral divisions of the diencephal, and that the "lateral recesses of the fourth ven- 

 tricle " may have a like relation to the metaccele or the epicrele, the mesocrele of birds 

 and frogs presents a distinctly triple constitution (Wilder, 63, 328, Fig. 4-6), with a mesal 

 iter and lateral optocoeles connected therewith by pylas, comparable with the division of 

 the prosocoelian cavity into the mesal aula and the lateral procreles connected therewith 

 by the portas. This serial homology is indicated in the following table : 



As a corollary to the above interpretation of each encephalomere as essentially mesal, 

 with or without lateral portions, it follows that the olfactory lobes are no longer recognized 

 by us as constituting a second, paired segment under the name rhinencephal, although 

 their cavities may still be conveniently called rhinocceles. Hence there are admitted only 

 five, definitive, encephalic segments instead of six, as in the first edition. 



The cerebellum and oblongata are regarded by Spitzka ( 16) as constituting a single 

 encephalomere, while, on the other hand, as stated more fully by Wilder (58), there are 

 reasons for thinking that several potential segments are more or less completely repre- 

 sented in this region. Pending the more complete determination of the number of these 

 segments, we believe the ordinary view is the most convenient and practical one ; that 

 between the myel and the optic lobes there are two encephalomeres, the metencephal, com- 

 prising the postoblongata, and the epencephal, including the cerebellum, the pons and the 

 preoblongata. But when it is needless or difficult to discriminate between them, the 

 entire region, as suggested by the senior author (63, 32) may be denominated metepencephal 

 and its cavity metepicale. 



(E) If the names of the principal, neural parts on the following Table are compared 

 with their common synonyms it will be seen that (1) most of them are correlated with 

 other names of parts in the same segment of the brain or having the same structure, and 

 (2) all of them are mononyms, consisting of a single word each. If they are compared 

 with the corresponding terms in 1058, it will be seen that in place of the Latin aspect, 

 most of them are, though Latin in general form, English in termination ; they are 

 paronyms of the Latin names ; mesoccdia, for example, is mesoccde, myelon and encephalon 

 become myel and encephal, etc. See preface to this edition. 



