300 MISS E. A. FRASER ON THE DEVELOPMENT 



obtained from Professor Hill's excellent collection of young stages 

 of mai'supials. 



Cavities in the mesoderm of the anterior (preotic) region of the 

 head, bounded by more or less definite epithelial walls, have been 

 described in many vertebrates, and they are now considered by 

 the majority of observers to represent somites comparable with 

 those of the trunk. They were first observed in Selachians 

 by Balfour (78), later by Milnes Marshall ('81), van Wijhe ('83), 

 Dohrn ('90) ('04), and since then by many other investigators 

 in different fishes. 



In Amphibians, cavities have been seen by Scott and Osborne 

 ('79) in the newt and in the Gymnophionan HypogeopMs by 

 Marcus ('09). They appear to be absent in NecUirus (Julia 

 Piatt, '94) ; in the frog (Corning, '99) and in the toad (Edge- 

 worth, '99) the anterior somites in the head are apparently 

 solid, although Goette ('75), in his classical work on the develop- 

 ment of fche toad, appears to refer to them as cavities. 



"Van Wijhe ('86) was the first to point out the occurrence of 

 head-cavities in birds and reptiles. In birds. Rex has more 

 recently stvidied them in the duck ('97) and in the gull ('01) 

 ('05), Edgeworth ('07) in the fowl, and Professor Hill has directed 

 my attention to the occurrence of large premandibular cavities 

 in sections of early emu chicks in his collection. 



The conditions in reptiles concern us more closely and may be 

 considered in greater detail. In this class three somites have 

 been observed in the preotic region of the head, of which the 

 first or premandibular develops into a large and conspicuous 

 cavity surrounded by epithelial walls, the cavities of either side 

 at some time of their development being usually connected across 

 the middle line by a transverse canal as in fishes. The second 

 and third somites are small and more diflacult to determine, and 

 observers differ considerably in their accounts of them. Van 

 Wijhe ('86) did not discover a second somite in Lacerta and he 

 describes the third somite as solid ; Hofimann ('90), on the other 

 hand, finds three hollow somites behind the premandibular, bvit 

 whether the last two both belong to the third head somite or 

 whether they correspond to somites three and four of fishes, he 

 was unable definitely to decide. Again in Lacerta, Corning ('99) 

 recognises the third somite but no similar second somite, the 

 m. obliquus superior, which usually develops from the latter, here 

 being described as arising from the doi'sal jDart of the primordium 

 of the trigeminal musciilature which grows out over the eye. In 

 Anguis fragilis, both the second and third somites were seen by 

 Oppel (90), each of which contained a small central cavity, but the 

 second was much less distinct than the third. In the Chelonia 

 the same somites are present, but whereas in Emys hctaria, 

 according to FilatofF ('07), the third is solid, in Chelydra serpen- 

 tina, described by Johnson ('13), both second and third possess 

 a distinct cavity round which the cells are arranged in a radial 

 manner. 



