520 



Prof. A. M. Paterson. 



[May 19, 



If, then, the question be asked, "What lies in front of the endo- 

 style ? " the immediate response is, " In both cases the proboscis- 

 cavity." 



I accordingly submit the following table of homologies : — 



(a.) Proboscis- cavity of Ascidians = Proboscis-cavity and prseoral 



pit of Amphioxus. 

 (b.) Endostyle of Ascidians = Endostyle of Amphioxus. 



(c.) Month of Ascidians = Month of Amphioxus. 



(d.) First pair of gill-slits of = First pair of gill-slits of 



Ascidians, in the improved Amphioxus. 



sense of the term. 



The homology of the clnb-shaped gland of Amphioxus with the 

 intestine of Ascidians, as suggested by van Beneden and Julin, would 

 seem, therefore, to be quite out of the question. 



It need hardly be pointed out that, if the homologies which I have 

 advanced are really correct, then the relations between Amphioxus 

 and the Ascidians become much less strained than they were on the 

 views previously entertained. 



I intend shortly to discuss the whole subject more elaborately in 

 the pages of the ' Quarterly Journal of Microscopical Science.' 



VI. " The Human Sacrum." By A. M. Paterson, M.D., Pro- 

 fessor of Anatomy in University College, Dundee, St. 

 Andrews University. Communicated by Professor D. J. 

 Cunningham, D.Sc. F.R.S. Received April 18, 1892. 



(Abstract.) 



Owing to the now classical investigations of Gegenbaur and 

 Frenkel, and the more recent researches of other observers, the several 

 homologies of the vertebral column are distinctly understood. The 

 specific or individual differences in the correlation of one region of 

 the column to another can be adequately explained on the assumption 

 of a suppression or excessive development of the potential costal 

 element of the vertebral segment. This costal element may be meta- 

 morphosed in different ways to suit the needs of the animal economy, 

 and the variations in individual cases affect the segments at the ends 

 of a series where the vertebrae of one region possess characters re- 

 sembling those of a neighbouring region. This hypothesis renders 

 intelligible, not only the existence of cervical ribs, but also correlated 

 variations of the thoracico-lumbar region, and abnormalities of the 

 sacrum, differences in the number of bones, as well as asymmetry. 



During recent yoars this aspect of the subject and numerous 



