222 
contiguous cells to each other, and gradually extends itself 
inwards towards the centre of each, leaving, however, the 
central nucleus and an investing layer of protoplasm unossi- 
fied ; and thus the so-called bone-corpuscles, which occupy 
the lacunse of bone, are formed. Whilst this is taking place, 
certain threads of soft protoplasm are left unossified, and 
these furm living threads of communication between neigh- 
bouring bone corpuscles. The channels occupied by these 
threads are the canaliculi of dried bone. After this process 
of hardening has extended so far as to transform a single 
layer of osteoblasts into a bony lamella, another layer of 
single cells, internal to the first, nndergoes a similar change; 
and thus a second lamella is formed. After the formation 
of several more lamellse in a similar manner has taken place 
in concentric series within the walls of a medullary space, 
no further deposit occurs, the central space still left being 
required for the vascular canal of the Haversian system. 
Simultaneously with the formation of Haversian systems, 
the osteoblastic cells which are formed beneath the inner layer 
of the periosteum are producing other lamellse parallel with 
the external surface of the bone ; and thus the bone grows 
in girth. This lateral growth takes place in membrane, and 
not in cartilage. Interstitial absorption is continually taking 
place in the dense substance of the bone through life, by the 
formation of fresh medullary spaces amidst the Haversian 
systems, and of new systems within such areas. This fact 
explains the peculiar intersection of the lamellae of different 
systems visible in a transverse section of mature bone, and 
also the greater number of such systems in the bones of old 
age. 
The paper was illustrated by drawings, and by a series of 
sections, showing the various stages of development of bone 
in man and in some of the lower animals. 
