Cliarles R. Bardeen 171 



ossification begin at about the time they take place in the vertebral bodies. 

 They are first seen in an area which corresponds to that in which the 

 nenral cartilage begins. The earliest calcification appears in Embryo 

 CLXXXIY, length 50 mm., in the arches of the first cervical to the sixth 

 thoracic vertebrae. 



The development of the ribs I shall not attempt in this place to describe 

 in detail. Figs. 35-34 and 37-42 show sufficiently well the relations of 

 the proximal ends of the ribs to the vertebrae. They are developed oppo- 

 site the intervertebral disks. The blastemal tissue which surrounds the 

 developing heads of the ribs becomes converted into costo-vertebral liga- 

 ments. Differentiation in the cartilage preliminary to ossification takes 

 place in the shafts of the ribs even earlier than in the vertebral bodies 

 and in the neural processes. Ossification is well under way in the shafts 

 of the ribs of Embryo LXXIX, length 33 mm. ; XCVI, length 44 mm. ; 

 XCV, length, 46 mm. ; and LXXXIV, length 50 mm. 



Summary of the Chondrogenous Period of Vertebral Development. — 

 Each cartilagenous vertebra is developed from four centers of chon- 

 drification. In addition, a separate center appears for each rib. In com- 

 paring these centers with the blastemal formative centers, we find that 

 each primative center of blastemal condensation enters into union with 

 tissue derived from the anterior half of the body-segment next posterior 

 and then gives rise to three centers of chondrification, one for the neural 

 arch, one for the rib and one for half a vertebra. When ossification first 

 takes place the centers for the ossification of the neural arches and the 

 ribs correspond to the original chondrification centers in the blastema, 

 but the centers for ossification of the bodies show little trace of the 

 bilateral condition which marks the cartilagenous fundaments. 



The processes of chondrogenous form differentiation are shown in 

 the drawings of the models. The period of ossification of the vertebras 

 has been so often and so well described that no attempt will be made to 

 enter upon a further ncount oE it in this paper. I have, however, not 

 found two primary ossification centers, such as Eenault and Eambaud 

 have described, for each neural arch. 



LITERATURE. 



Bade, P. — Die Entwicklung des menschl. Skelets bis zum Gebiirt. Arcliiv 



mikr. Anat., LV, 245-290, 1900. 

 Baldus. — Die Intervertebral Spalte v. Ebners und die Querteilung der 



Schwanzwirbel bei Hemidactylus mabuia. Dissertation, Leipzig, 1901. 

 Bardeen. — Development of the Musculature in the Body-wall of the Pig. 



Johns Hopkins Hospital Reports, IX, 367, 1S99. 

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