420 TRANSACTIONS OF THE CANADIAN INSTITUTE, [VoL. VI 
still no nuclear membrane and still no iron-ho!ding nuclein compounds 
in the cell body or process. The cell bodies appear, quite frequently, 
reticulated. 
The equatorial-plate stage is soon passed, and the chromatin begins to 
distribute itself in the nucleus. | Various steps in the distribution of the 
chromatin may be followed until a stage, such as is represented in Fig. 
15, is reached. By this time a nuclear membrane has been formed. 
Usually several masses of chromatin are found touching the nuclear 
membrane, while others are found towards the centre of the nucleus. but 
all seemingly connected by filaments. There is, as yet, no oxyphile 
substance in the nucleus. These cells are usually found under the 
membrane of the medullary canal but in very young embryos they occur 
in the mantle layer. The nucleus is entirely excentric and the cell body 
runs out into a long process. The cell body and process are still 
entirely free from iron-holding nuclein compounds. 
As the cells pass outward into the mantle layer and become older, 
the substance having affinity for toluidin blue in their nuclei disappears 
and a substance with more affinity for eosin takes its place. Syn- 
chronous with this change, a substance with great affinity for toluidin 
blue appears in the cell around the nucleus (Figs. 16 and 17). With 
the appearance of this substance in the cell body iron may be de- 
tected there for the first time. In this stage, which would correspond 
to stage 4 of the series described by His, there are several granular 
masses in the nucleus with marked affinity for toluidin blue, but the 
most of the nuclear chromatin stains intermediate between the red and 
the blue. 
As development proceeds (Figs. 18 and 19) the basophile substance in 
the nucleus continues to decrease, while the basophile substance in the 
cell body increases, and as it does so the affinity of the nuclear chro- 
matin for eosin also increases correspondingly. One part of the chro- 
matin does not alter but remains basophile and constitutes ultimately the 
peripheral portion ofthe nucleolus. Figs. 18 and 20 represent cells from 
the same embryo. The one indicates the distribution of the oxyphile 
and the basophile parts, while the other shows that both contain iron. 
In Figure 21 is represented a cell from the medulla of a 32mm. 
pig embryo. The basophile substance forms a homogeneous mass filling 
the cell body. The cell may be said to have undergone at this 
time complete development of its chromatic substance, for now the 
nuclear oxyphile substance is completely digestible, and it stains like the 
substance found in the nuclei of adult mammalian nerve cells. 
