Franklin P. Mall 345 



cartilage, and occasionally in other cartilages of the head as well as in 

 the ribs. Otherwise the boundary line between the cartilage and the 

 surrounding connective-tissue syncytium is quite sharp and is obliter- 

 ated by the dense tissue and nuclei of the perichondrium. Specimens 

 stained by Mallory's method are the best by all odds for studying the 

 transition of syncytium into cartilage, for in them the ground substance 

 and exoplasm are stained intensely blue, while the nuclei and endoplasm 

 are shrunken and tinged red. 



After the cartilage is once well formed its further growth is intersti- 

 tial as well as peripheral. Not only do the nuclei divide, but the ground 

 substance increases out of proportion, forcing the nuclei apart. This 

 is beautifully illustrated in the sheath of the chorda, which is gradually 

 incorporated with the vertebral cartilage. Often the thickened sheath 

 appears as a great stump with its roots extending out into the cartilage. 



In an embryo 3 cm. long the occipital and petrous cartilages have ex- 

 tended greatly, but still show beautifully the connection of the ground 

 substance with the exoplasm of the syncytium. The transitions from 

 the nuclei, endoplasm and exoplasm, into cartilage are again as distinct 

 as in embryos 2 cm. long. This indicates that the syncytium at the 

 peripher}'' is being changed into and added to the cartilage already 

 formed. 



In older embryos the cartilage becomes separated more and more from 

 the surrounding syncytium, Avith the exception where the cartilage is 

 still extending into it. In an embryo 5 cm. long this condition is still 

 present in the occipital cartilage and in the sternum. The borders of 

 the vertebra and most of the other cartilages are well defined and are 

 beginning to ossify at different points. 



Bone. 



The frontal and mandibular are the first membranous bones to appear 

 in embryos about 2 cm. long. In smaller embryos no indication of 

 the formation of bone can be seen. Sections through the frontal region 

 of embryos 2 cm. long stained by Mallory's method shoAV that the bone 

 begins by a very blue zone of hyaline deposit in the exoplasm of the 

 connective-tissue syncytium (Fig. 11). The deposit appears to be 

 equally distributed throughout the exoplasm within this zone. The nuclei 

 stain somewhat more intensely than those of the surrounding syncy- 

 tium, and the endoplasm around them is increased in quantity. Nuclei 

 and endoplasm now show all the characteristics of osteoblasts and may 

 be considered such. Sections certainly show definitely that when the 

 syncytium turns into bone the nuclei become more sharply defined, the 



