Franklin P. Mall 347 



zone of osteoblasts, where there are transition forms between them and 

 the nuclei of the syncytium, there are heavier fibrils of bone radiations 

 between the nuclei. Passing from these fibrils towards the finished 

 bone it is seen that they soon unite to form bundles which are soon 

 stuck together to form the apparently homogeneous bone substance. 

 After the bone is once well formed the primitive Haversian canals are 

 filled with osteoblasts and a core of connective-tissue syncytium in 

 which course the blood-vessels. 



The difference in appearance between the beginning of bone in 

 embryos 3 and 5 cm. long is probably due to the perfect syncytium in 

 the former and the syncytium differentiating into prefibrous tissue in 

 the latter. In the first the bone is deposited directly in the exoplasm, 

 while in the second the exoplasm is first partly changed into prefibrous 

 tissue and then into bone. 



There are marked changes in the cartilages of embryos 5 cm. long, 

 preparatory to their ossification. The shafts of the clavicle and ribs 

 are encircled with a shell of bone and the transverse processes of the 

 vertebra are beginning to ossify. In suitable sections of the latter it 

 is seen that the perichondrium is thickened and filled with osteoblasts. 

 Not only do the osteoblasts appear to be arising from the nuclei of the 

 perichondrium, but also from the outer layer of cartilage cells. Between 

 the osteoblasts the bone fibrils first appear; they stain intensely blue 

 with Mallory's stain, more so than the ground substance of the cartilage, 

 extend throughout the perichondrium and extend somewhat into the 

 cartilage. In order to separate the bone fibrils from cartilage it is 

 necessary to stain sections with hsematoxylin and congo red. Such 

 sections show that the greater part of the bone is first deposited in the 

 perichondrium, and only a small amount, if any, in the ground substance 

 of the periphery of the cartilage, in periosteal ossification. 



So it appears that in periosteal ossification the connective-tissue syn- 

 cytium changes partly into cartilage and partly into white fibrous tissue 

 before it gives rise to bone. Possibly the study of some suitable sec- 

 tions from embryos smaller than 5 cm. will give results identical with 

 those obtained for the frontal bone, but so far I have been unable to 

 obtain such sections. 



White Fibeous Tissue. 



In the study of the development of cartilage and bone definite spots 

 .can be located and followed from stage to stage. To do the same with 

 white fibrous tissue and the other connective-tissue fibrils is much more 

 difficult. Finally, after trying many regions, I settled on the develop- 



