PLATE III. 
Fic. 20. Outline drawing of the fifth operative stage (XII). The gill mass 
shows three divisions, and the tentacles are beginning to develop. xX 6% 
diameters. 
Fic. 21. Section through eye region of an embryo of above stage (XII). 
The lens-vesicle is divided off from the ectoderm, but still in close contact 
with same. X 45 diameters. 
Fig. 22. Experiment XII;,. Section through right lens-vesicle of an em- 
bryo killed 2 hours after the complete extirpation of the optic cup, showing 
no special changes within this time. X 180 diameters. 
Fig. 23. Experiment XII,;, Section through left normal lens of above 
embryo, killed 2 hours after the operation. X 180 diameters. 
Fig. 24. Experiment XII,,. Section through the right lens-vesicle of an 
embryo killed 2 days after removal of the optic cup. The lens is similar to 
the normal one in size, 150 in diameter, and in differentiation. x 180 
diameters. 
Fie. 25. Experiment XII,,. Section through left normal lens of above 2 
day embryo. X 180 diameters. 
Fie. 26. Experiment XII,. Section through right abortive lens 12 days 
after removal of the influencing optic cup. A complete layer of cuboidal 
epithelium surrounds the central mass of degenerating lens-fibers, and has 
obliterated the medial pole entirely. > 180 diameters. 
Fic. 27. Experiment XII;,. Section through left normal lens of a 6-day 
embryo. XX 180 diameters. 
Fic. 28. Experiment XII,,. Section through right lens of the above embryo 
killed 6 days after the complete extirpation of the optic cup. It is separated 
from the ectoderm by mesenchyme, and the medial pole is almost obliterated 
by the overgrowth of the epithelial layer. X 180 diameters. 
