214 A. B. MACALLUM. 
in some cases, in such abundance as to obscure the outlines of 
the trabecule. He has found that in the fibro-vascular bundle 
this end of the nucleus is directed toward the base of the 
ovule, and is of the opinion, as a result of some investigation 
of this subject, that the phenomenon in question is connected 
with the processes of the formation of chromatin, which he 
regards as taking place here. 
The presence of assimilated iron, apart from its occurrence 
in hemoglobin and hematin, is an exceptional feature in the 
cytoplasm of the cells of the higher forms of animal life, but 
the exceptional instances are themselves of a constant cha- 
racter, and comprise, in addition to yolk-holding ova, the cells 
of yolk-holding embryos, the hematoblasts of Vertebrates, and 
the ferment-forming gland-cells of all descriptions. 
The iron in the yolk of Amphibian ova is held in the yolk- 
spherules, which manifest a strong affinity for dyes, and are 
usually homogeneous in composition. These give with ammo- 
nium hydrogen sulphide a dark green reaction, which makes 
its appearance sometimes in a few minutes, but at the latest in 
a few hours, when the preparation is kept warm. The reaction 
is uniform throughout each spherule. The enclosing cyto- 
plasm does not, before development of the ovum begins, con- 
tain any assimilated iron; but in the developing embryo, with 
the multiplication of the cells and the partition of the yolk, 
the spherules gradually undergo solution, for they become 
smaller in size, and then one obtains an iron reaction in the 
cytoplasm of each cell. The solution of the yolk-spherules may 
be studied also in preparations made with the carmine-indigo- 
carmine fluid, the reagent giving, in the earliest stages of the 
embryo, a green colour to the yolk-spherules, and a red stain 
to the cytoplasm and nuclei; but in later stages the red colour 
is rarely obtainable, and both cell and nucleus, the latter espe- 
cially, are coloured blue-green or dark green. This result is 
brought about by the solution of the yolk-chromatin in each 
spherule and the diffusion of the dissolved substance through 
the cytoplasm and nucleus of each spherule-holding cell, for 
in those examples of larval Amblystomata which yield pre- 
