POLYGYEA ALBOLABRIS AND LIMAX MAXIMUS 309 



PLATE 25 



This plate shows the gradual development of the embryo from the ger- 



minatlve vesicle. 

 Fig. 



1 The germinative vesicle within the egg, natural size and x 8. 



2 Germinative vesick, x 16. 



3 Under side of same at the commencement of segmentation, 



X 16. 



4 The segmented ovum, x 16. 



5 The endodermal cell mass, Inclosed by a transparent sac or 



wall of ectoderm, x 16. 



6 Showing the transparent sac greatly developed on one side 



of the cell mass, x 16. 



7 The beginning of the development of the body form, as 



shown by a slight elevation from the cell mass, x 16. 



8 A stage slightly more advanced, x 16. 



9 The elevation is divided into two lobes, representing the 



body proper and the shell sac, x 16. 



10 The embryo within the egg, natural size and x 8. 



11 A stage more advanced than in figure 9. The podocyst is 



here plainly shown, x 16. 



12, 13 Still more advanced stages, x 16. 



14 An embryo within the egg, x 8. 



15 In this stage the tentacles are rapidly developing, the body 



and podocyst are distinct, and the development of the 

 buccal body and alimentary canal has begun, x 16. 



16-19 Gradual development of the embryo. In figure 16 the en- 

 dodermal cell mass has reached its greatest size, gradu- 

 ally becoming absorbed in succeeding stages, x 16. 

 18 The embryo within the egg, showing the same stage as in 

 figure 19, X 8. 



20, 21 Ventral and profile views of the embryo within the egg, 

 X 8; showing a stage more advanced than that repre- 

 sented by figure 19. 



