588 MOEPHOLOGICAL METHOD AND RP^CENT PROGRESS IN ZOOLOGY. 



To commence with the Mammaliii, we now know that the mammaiy 

 o-land when tirst it appears is in all fornjs tnl>ular, and that this t3"pe 

 is no longer distinctive of the Monotremata alone. We know, too, 

 that the intranarial position of the epiglottis when at rest, long- known 

 for certain forms, is a distinction of the class. It explains the pres- 

 ence of the v^clum palatinum by its association with the glottis for the 

 restriction of the respiratory passage, the connection being lost in 

 man alone under specialization of the organ of the voice. 



Similarl}' the doubly ossified condition of the coracoid ma}^ now be 

 held diagnostic, for it is known that the epicoracoidal element, origi- 

 nally thought to characterize the monotremes alone, is always present, 

 and that reduction to a varying degree characterizes the metacoracoid, 

 which retires, as in man, as the so-called coracoid epiphysis. 



Our conceptions of the interrelationships of the Marsupialia and 

 Placentalia have, during the period we are considering, been delimited 

 bej'ond expectation by the discovery of an allantoic placenta in a poly- 

 protodont marsupial in place of the vitelline, present in its allies. 

 When it is remembered that in the formation of the placenta of the 

 rabl)it and a l)at there is realized a provisional vitelline stage, it is 

 tempting to suggest that the evidence for the direct relationship of 

 the two mannnalian subclasses first named overlaps (there being a pla- 

 cental marsupial on one hand, a marsupial placental on the other), 

 nnu-h as we have come to regard Archwopteryx as an avian reptile, 

 the Odontornithes as reptilian birds. These facts, moreover, prove 

 that the type of placenta inherited by the Placentalia nuist have been 

 discoidal, and that from that all others were derived. 



Equally important concerning our knowledge of the Marsupialia is 

 the discovery, first made clear by Professor Symington, of this col- 

 lege, that Owen was correct in denying them a corpus callosum. How 

 Owen arrived at this conclusion it is diificult to conceive, but in these 

 later days the history of discovery is largely that of method, and it is 

 by the employment of chrome silver, methylene blu(>, and other 

 reagents, which in dift'erentiating tlie fil)er tracts enable us to delimit 

 their course, that this conclusion has been proved. By the corpus 

 callosum we now understand n series of neopallial fibers which tran- 

 sect the alveus and are present only in the Placentalia. 



There is no department of mammalogy in which recent work has 

 been more luminous than this which concerns the brain, and, to men- 

 tion but one residt, it may be said that in the renewed study of the 

 conunissures there has been found a fil)er tract characteristic of the 

 Diprotodontia alone, so situated as to prove that they and the Placen- 

 talia must have specialized on diNcrse lines from a polyprotodont 

 stock. Interesting this the more, since the phalaiigers and kangaroos 

 are known to l)e polyprotodont when 5^oung. And when we add the 

 discovei-y that in the detailed relationship of its conunissures the l)rain 



