1907] GATES—POLLEN DEVELOPMENT AND MUTATION 113 
Fic. 5. A loculus after the pollen mother cells have disappeared; the rem- 
nants of the tapetum form a thin layer partly lining the cavity; the ingrowth 
of the middle layers has begun. X 325. 
Fic. 6. A later stage; the middle layers have grown in, almost completely 
obliterating the loculus. x 325. 
1G. 7. Late stage in pollen development; the middle layers are being pressed 
together; the tapetal cells are narrow radially; the pollen grains are very irregular 
in shape. X155. 
Fic. 8. ‘ Resting” nucleus of pollen mother cell; the nucleolus contains several 
highly refractive vacuoles; at the nodes of the reticulum are chromatic bodies, 
of OVERTON. X 2125. 
Fic. 9. Slightly later stage; the chromatic bodies are rounded in shape; 
siideeniin. eae 3:3. Aares: 
Fic. to. Slightly later stage; the reticulum is beginning to contract away 
from the nuclear membrane, which at this time is extremely delicate; 3.3 x. 
X 2125. 
Fic. 11. Spirem stage just previous to synapsis; four nucleoli are present; 
the nuclear membrane is extremely delicate; the spirem thread is very thin, 
ut half its thickness subsequent to synapsis; Safranin-gentian; 4 H- 
X 2125. 
Fic. 12. Spirem thread contracted in synapsis; iron-haematoxylin-orange; 
5é@. X 2125. 
Fic. 13. Mother cell in synapsis; chromatic masses are scattered through the 
Fic. 14. Just after synapsis; loops of the spirem pass outwards to the wall, 
giving a false appearance of doubling; the nucleolus is flattened and appressed to 
the nuclear wall, which is more definite at this time; iron-haematoxylin-orange; 
SM& X2125. 
PLATE III 
Fics. 15-24. O. lata (dominant). 
Fic. 15. Nucleus at the time of synapsis; the nuclear membrane is very thick 
and irregularly indented; safranin-gentian; 4“. X2125. 
Fic. 16. Portion of spirem after synapsis; it is about double its former 
thickness and very much shorter; the loop (a) of the spirem may indicate the 
method of origin of a heterochromosome; safranin-gentian; 4#. X2125. 
Fic. 17. Slightly later stage showing a portion of the spirem after synapsis; 
the large ring body, which has been detached from the spirem, is a hetero- 
chromosome (k); the nucleolus is present and the nuclear membrane is becoming 
delicate previous to breaking bey Lanoenehannent 3. 3 K. Xa 2125. 
Fic. 18. Section of nucl liand a het (A); 
the cna section contains the spirem; safranin-gentian; 4#. 2125. 
G. 19. Similar section showing two heterochromosomes (/:) and the nucleo- 
lus; erihhehet cor 4B. X2125. 
