258 Proceedings of Royal Society of Edinburgh. [sess. 
The Development of Diarthrodial Joints in Birds and 
Mammals. By David Hepburn, M.B., M.B.C.S. (Eng.), 
Senior Demonstrator of Anatomy , University of Edinburgh. 
Communicated by Professor Sir W. Turner. 
(From the Embryological Laboratory, University of Edinburgh.) 
(Read May 20, 1889.) 
After giving a summary of recent literature on the subject, the 
author then proceeded to state the nature of the material which he 
had employed in the present investigation. 
The bird selected was the common fowl ( Callus domest.), and he 
had examined a series of microscopic sections through the limbs 
from the fourth day of incubation to the day of hatching. 
The mammalian embryos examined were mice and rabbits, and 
the fingers of the human foetus from an embryo approaching the 
full period of uterogestation. 
Method of Preparation. — The embryo chicks were prepared, 
partly by hardening in picrosulphuric acid and partly in dilute 
solutions of nitric acid. The human embryos were also hardened in 
nitric acid. The embryos were then dehydrated with alcohol, stained 
in borax-carmine, and cut with the Cambridge rocking microtome, 
the average thickness of the sections being *006 mm. 
The author expressed his indebtedness to Mr George Brook, 
Lecturer on Embryology in the University of Edinburgh, under 
whose guidance and in whose laboratory the investigation had been 
conducted, and then proceeded to give a summary of the results 
which he had attained. 
At the end of the fourth day of incubation the wing of the chick - 
is in the form of a bud, '8 mm. long, and consists of a mass of 
mesoblast cells enveloped in a covering of epiblast. At this stage 
the cells of the mesoblast present no differentiation into separate 
structures, but at the end of the fifth day the free extremity of the 
limb has assumed a bulbous form, and horizontal sections show that 
along certain lines a process of condensation has occurred, apparently 
presaging the positions of future bone matrices. The individual 
cells in these portions present no great change from the rest of the 
