57 



entertained by Wagner, Bisclioff, Barry, and Wharton Jones, con- 

 cerning the membranous nature of that portion of the ovule known 

 as the zona pelliwida. 



6. That the oil-globules of the yelk are either enclosed in a di- 

 stinct membrane, or else that a structureless solid material pervades 

 the entire substance of the vitelline body, and so binds the several 

 component elements of it together. 



7. That the recognition of the germinal vesicle removes some 

 doubts concerning its appearance and position in the germ-mass. 



May 8, 1851. 



The EARL OF ROSSE, President, in the Chair. 



Professor Owen delivered the Croonian Lecture, being the sub- 

 stance of his paper " On the Megatherium." — Part II. Received 

 May 6, 1851. 



In his lecture the author premised a brief sketch of the successive 

 steps which had led to the knowledge of the Megatherium acquired 

 at the date of his researches, and of the different hypotheses which 

 had been broached of its affinities, habits and food. He then recounted 

 the mode of the acquisition of the complete skeleton, and of its articula- 

 tion, at the British Museum, and commenced its description by the 

 vertebrae of the trunk. These consist of 7 cervical, 16 dorsal, 3 lum- 

 bar, 5 sacral, and 18 caudal vertebrae. The first to the fifth dorsal 

 vertebrae are characterized by having the ordinary number of arti- 

 cular processes (zygapophyses), two before and two behind ; and by 

 having three articular surfaces for the ribs on each side, one on the 

 centrum, one on the neurapophysis, and one on the diapophysis. 

 The sixth dorsal vertebra has an accessory zygapophysis between 

 the posterior pair ; the thirteenth dorsal has one between the anterior 

 pair ; the seventh to the twelfth inclusive have the accessory median 

 zygapophysis between both the anterior and posterior pairs of the 

 ordinary zygapophyses. The fourteenth and succeeding dorsals 

 have no costal surface on the diapophysis or centrum. The fif- 

 teenth has both metapophysis and anapophysis — the latter with an 

 articular surface : the sixteenth superadds the parapophysis with an 

 articular facet. 



The lumbar vertebrae lose the costal surface on the centrum, and 

 retain the metapophyses, anapophyses and parapophyses. The 

 nature of these accessory processes was explained by reference to 

 the descriptions and figures of the exogenous processes of vertebrae 

 in Part I. of the present Memoir. 



The characteristics of the cervical vertebrae were next detailed. 



Of the five anchylosed sacral vertebrae, three are confluent with 

 the iliac bones, and two with the ischia. 



The fourteen anterior caudals are characterized by articular sur- 

 faces for haemapophyses. These elements are separate from each 

 other in the first caudal, and confluent as usual at their distal ends, 

 forming a ' chevron-bone' in the others. The posterior zygapophy- 

 ses lose their articular surfaces in the eleventh caudal ; the anterior 



