120 C. M. Jackson. 



also the volume may be determined, where it is difficult or, impossible 

 to ascertain the weight. Even where the organs are large enough 

 to be dissected out and weighed, one does not like to sacrifice valu- 

 able specimens for this purpose, if it can be avoided. On the other 

 hand, it is comparatively easy, though somewhat tedious, to measure 

 the volumes of embryos which have been cut into serial sections. The 

 sections must first be drawn to a definite scale of enlargement. Then 

 one may proceed in either of two ways. In the first embryo meas- 

 ured (11 mm.) a rough model was constructed by Bom's wax-plate 

 method, and the volumes of the body and of the various organs and 

 parts were measured by water displacement.^ An easier method,^ 

 which is equally accurate, was used with other small embryos. In 

 the enlarged drawings of the sections, the areas of the body and of 

 the various organs were measured by means of a planimeter. The 

 volumes desired were then easily calculated by multiplying the areas 

 (corrected for magnification) by the thickness of the sections. From 

 the third month onward, it was found possible to measure directly 

 the volumes desired by means of water displacement in gradu-ated 

 glass cylinders of various sizes. In some fresh specimens the volume 

 and specific gravity were determined by weighing successively in air 

 and suspended in water. 



Certain sources of error must be recognized in the use of these 

 methods. First is the effect of the reagents used. Most of the speci- 

 mens used had been fixed and preserved in 5 per cent to 10 per cent 

 formalin solution. It is well kno^vn that in general formalin causes 

 a certain amount of swelling or expansion of tissues. In one fetus 

 of the 5th month in which this point was carefully observed, the 

 swelling amounted to nearly 13 per cent of the total volume, after 

 three months in a 10 per cent formalin solution. Furthermore, it is 

 stated that the swelling is not equal in all of the various organs, 

 though the amount of difference and the conditions of occurrence are 

 not yet fully known. Alcohol, on the other hand, causes shrinkage. 



'This embryo was studied by Bonnot and Seevers (6) under my direc- 

 tion. I am also under obligations to J. A. Watkins, M. L. Clint, and R. 

 Lhamon for assistance in making a part of the observations used in this, 

 paper. 



