I.] THE FROG. 129 



one nucleus may be present (resting cell) or two 

 (dividing cell). 



ft. A two-celled nest ; one or both of the cells may 

 lodge two nuclei. 



y. A three-celled nest. Usually one cell will be 

 found to be the larger of the three ; its nucleus 

 may or may not show traces of division. 



5. Bone. 



a. Cleave the femur of a recently-killed frog longi- 

 tudinally in two, and examine while still fresh. 



a. The bone ; its central cylindroidal shaft and 

 terminal epiphyses (cf. p. 14). 



ft. The marrow ; a fatty vascular mass, filling the 

 central cavity. 



y. The periosteum ; a tough vascular connective- 

 tissue sheath, closely investing the shaft. 



B. The nutritive foramina. Carefully remove the 

 marrow and examine from within; the torn 

 ends of the periosteum will be seen passing in 

 through the above, for communication with the 

 marrow. 



b. Decalcify the middle third of the femur of a freshly- 

 killed frog in \ per cent, chromic acid solution, 

 stain with borax-carmine and cut into transverse 

 sections as directed in the Appendix. Mount in 

 Canada balsam, and examine under a low power. 



a. The bony shaft ; seen to be divided, by a highly 

 refractive middle lamella^ into an outer more 

 deeply stained portion and an inner thinner and 

 less deeply stained one. 



M. 9 



