Pratt. — Imaginal discs. 
PLATE 4. 
v 
Fig. 23. Parasagittal section of the same embryo as that shown in the preced¬ 
ing figure. X 130. 
Fig. 24. Cross-section through the extreme anterior end of an embryo some¬ 
what older than that shown in the two preceding figures. X 200. 
Fig. 25. Cross-section through the base of thfe sucking tongue of the same 
embryo. X 200. 
Fig. 26. Cross-section through the cephalic discs of the same embryo. X 200. 
Fig. 27. Surface view of a wax model representing the anterior end of an 
embryo of about the same age as that shown in Figs. 22 and 23. X 300. 
Fig. 28. Surface view, lateral aspect, of the head end of an embryo of the 
same age as that shown in Figs. 22 and 23. X 200. 
Fig. 29. A slightly oblique sagittal section of an embryo, older than that 
shown in Fig. 22, in which the involution of the head has begun. 
X 130. 
