196 DEVELOPMENT OF THE MARSUPIAL SKULL, 



Procavia, Gallus, Struthio, Chanieleo, Agama, Scajyteira, EremiaSf. 

 Zonurus, Sphenodon and Teshido among the liigher vertebrates^ 

 but one or two points in the marsupial structure still remain 

 obscure. I think it advisable, however, at this stage to publish 

 an account of the facts of development, leaving to some later date 

 the clearing up, if possible, of those points whose interpretation 

 is doubtful. 



The marsupial which I have chiefly studied is the diprotodont, 

 Trichosurus vulpecula, of which I have a very complete series. 

 A short account of the external and, to some extent, of the 

 internal characters of the embryos, with figures of the more 

 important specimens, was published some years ago by this 

 Society. Through the kindness of my friends, Professors J. T. 

 Wilson and J. P. Hill, I am enabled to give an account of an 

 interesting early stage of the polyprotodont Dasyurus viverrinus. 



Trichosurus vulpecula. Stage i. 



Four embryos which may be regarded as belonging to this stage 

 have been cut and examined. These are the embryos referred ta- 

 in the above-mentioned paper as A, B, 0, and E; and they vary 

 in length from 8-5 mm. to 11 mm. In embryo A, only the back 

 part of the head has been preserved, but the degree of chondri- 

 fication does not differ greatly from that of B and C. In these 

 latter two the cranium is well preserved, and shows very satis- 

 factorily the early condition. The embryos are about the same 

 size, and there are only a few points of difference in the degrees 

 of development. Embryo E is a little in advance of the other, 

 but not greatly. The following description of the condition of 

 Stage i., is taken from embryo C, unless otherwise stated. 



The skull at this stage is particularly interesting and, in one 

 or two respects, unlike what was expected from Parker's work on 

 the pig. The cranial axis is fairly well chondrified. In the occipital 

 region it is present as a broad, curved sheet in which there are 

 two foramina, both apparently for branches of nerve xii. A 

 little in front of this region the basal cartilage becomes much- 

 narrowed to accommodate the auditory capsules. Between the- 



