218 M. IXABA. 



there the distinct groupiiigs of its cells into cords (tig. 19), where 

 the nuclei are larger and the protoplasm is less stained, than in the 

 adj(jining cortical cells. This stage seems to be the formation of the 

 medullary cords. The three minor zones of the cortex are already to 

 he found, thouoh less distinct than in the staii'e described before. 



In the mouse three da^^s old (hg. 1(5). the medulla is very 

 irregular in its outline. Alon£>' its maro-in the cells are "'reatlv 

 minüded with the cortical cells, luit the distinction is cleai-, the cells 

 and nuclei of the medulla being stained more deeply and packed more 

 closely, than in the cortex. The three minor cortical zones are not 

 yet distinguishable. 



In the newly-born nKjuse (wood-cut 1 and PI. XXXI. hg. 15), 

 the medulla no hunger forms any compact mass, but has cortical cells, 

 intermixed throughout its substance. The distinctions between the 

 two substances can however be easily made out as before. 



The relative size of the nuclei in the two substances is interesting. 

 In hgs. 15 and 16 (PI. XXXL), the nuclei of the medullary cells are 

 evidently smaller than those of the cortical cells, while in tig. 21, the 

 case is reversed. I measured the nuclei of cells in the two sub- 

 stances near their boundary line at various stages. The following 

 gives the average size (in u) of those nuclei. 



1 day old. 3 days. 10 days. 29 days. adult. 



Medulla 5.2— 5.6— :^.Q+ (5— 6 — 



Cortex G.5 — 6. — 5.4— 5 — 5 -f- 



It will be seen from the tal)le that for about a month after birth, the 

 cortical nuclei are gradually decreasing in size ; at the same time the 

 medullary nuclei are growing though very slightly. This is, I believe, 

 due to the formation of the cord-like arrangement on the part of the 

 medulla, and of the z<_)iia reticulata <_)n the part of the cortex. 



