194 
EXPLANATION OF PLATE XVII 
Fig. 22. Section through the closed labella in the plane where the 
free end of the axial apophysis (q) first appears. In this 
section the teeth have disappeared and the labella is 
closed ventrally. x 750. 
F'g- 2 3 - Section showing the fusion of the axial apophysis (q) with 
the tooth plate (k). x 7150. 
Fig. 24.—Another phase of the same. x 750. 
I ig. 25. Section through the closed labella in the plane at the base 
of the axial apophysis (7). Here the tooth plate has 
entirely disappeared ; the arms of the fork are seen con¬ 
verging, and stretching from tip to tip is a broad 
membrane ; here also are seen the lateral sclerities of the 
fork (<d) articulating with that organ, x 750. 
