4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 'J2. 



CROSS SECTION OF ANOTHER PORTION OF THE LOWER JAW OF THE MAS- 



CALONGE SHOWING THE EARLY DIFFERENTIATIONS OF 



HAVERSIAN SYSTEMS 



Pl. I, Fig. 2 

 These are small vascular canals surrounded by clear areas — in 

 some instances lamellated, in others, homogeneous. Between the 

 vascular canals is the bone substance with very fine channels. The 

 vascular canals with surrounding clear areas become Haversian sys-. 

 terns as vascular differentiation progresses. This early form of 

 Haversian differentiation has a wide distribution since it is found in 

 bone sections from fish to and including man. It is much more prom- 

 inent in the lower orders of vertebrates than in the higher. Thus 

 two early phases of the circulation are found in the bone of the 

 mascalonge, viz., the channeled and early Haversian differentiation. 



TANGENTIAL SECTION OF THE INNER RIDGE OF THE LOWER JAW OF 

 THE MASCALONGE SHOWING THE BLOOD VESSELS 



Pl. I, Fig. 3 

 The blood vessels vary in size. They are parallel with each other. 

 Some are branching. Around the outside of their delicate connective 

 tissue walls are seen fine plexuses of nerves. Between the vessels 

 is seen a fine channeled bone substance without lacunae. 



CROSS AND LONGITUDINAL SECTIONS OF THE FEMUR OF A 

 MEDIUM SIZED BULLFROG, RANA CATESBEIANA 



(The large, medium sized and small bullfrogs have different degrees of bone 



differentiation.) * 

 CROSS SECTION SHOWING THE STRUCTURE 

 Pl. I, Fig. 4 

 The type of bone is an early first. The inner wall is composed of 

 bone substance in which are radiating, vascular, bush-like channels 

 extending from the external to the internal circumferential lamellae 

 and between which are round or oval lacunae with short branching 

 canaliculi embedded in the bone substance and communicating with 

 the bush-like radiations by means of their canaliculi. The bush-like 

 vascular radiations appear to be segmented in some instances due 

 to their oblique positions and the plane of the cross section. In the 

 outer wall these radiating channels are absent and the bone is com- 

 posed of lamellae with oval lacunae and radiating canaliculi. 



— — ^ 



* Idem. 



