2,6 FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. i. 



ination of cross and longitudinal sections that the bony deposits found 

 at the base of each half arch do not at first extend all around the 

 base. The bone is wanting both in front and behind the base; neither 

 does it pass between the base and the elastica. 



While it is possible to regard these centers of bone growth as 

 'common to the arches an,d to the tissues in which the primitive ring 

 is developed, as Rathke thought, they appear to appertain more 

 properly to the arches. On their first appearance they extend fur- 

 ther up on the arch than they do along the circumference of the no- 

 tochord, and in many instances they are seen hardly to touch the lat- 

 ter. In other words, the centers belong to the arches rather than to 

 any tissue in closer relations with the notochord. However, the little 

 deposits of bone soon- begin to extend themselves over the notochord 

 more rapidly than along the arches, and then they appear to belong 

 rather to the former. 



The bone is perichondrial in its origin where it is in connection 

 with the cartilage of the arches. It forms at first an extremely thin 

 layer wholly devoid of any bone-cells, but as the layer thickens these 

 cells are seen to be included. When, however, the bone lies against 

 the membrana elastica, the bone cells are included from the start. 



Through the extension of the various centers of each vertebra 

 around the notochord and their union with one another, a continuous 

 ring of bone is formed. Meanwhile, the two centers at the base of 

 each arch have grown forward and backward and met, and the deposit 

 continues to advance until it covers the whole centrum with a bony 

 crust. In a larva 3omm. long, ossification has so far advanced in the 

 dorsal region as to join all the centers into one band, which has 

 attained nearly the full length of the centrum ; while in the middle 

 of the tail only a tiny flake of bone is to be seen on each side of the 

 base of each half-arch. 



Soon after the vertebral band has closed around the notochord 

 the bone begins to push itself out into the connective tissue. The 

 new deposit is in the form of narrow bars which shoot out at right 

 angles from the primitive ring, and these again become united by 

 other bars more or less parallel with the ring. A loose meshwork 

 of bone is thus finally produced, such as is to be seen in the adult 

 vertebra. This network of bony spicules is first seen between the 

 bases of the lower arches of the dorsal region in the loose connective 

 tissue just above the aorta. 



In the case of the centers of ossification belonging to the bases 

 of the lower arches in the trunk region, I find the bone appearing on 

 the lower surface of the cartilage, both mesiad and laterad of the 



