EXPLANATION OF TLATE lY. 



Figs. 39 to 45. Sagittal sections tlirougli the tail-fold or posterior end of 

 embryos, showing the migration of the germ-cells within the body, and their 

 crowding together to form the germ-glands. Figs. 39 and 42, x 1^5; the 

 others, x 140. 



Fig. 39. Embryo to Stage K (39). Almost all of the pole-cells have pene- 

 trated into the embryo. 



Fig. 40. Embryo in Stage L (40). a, two pole-cells lying in the much 

 shortened pole-cell canal. 



Fig. 41. Embryo in Stage M (41). The germ-cells are closer together than 

 in a younger embryo (Fig. 40). 



Fig. 42. Embryo in Stage N (42). The germ-cells form a distinct germ- 

 gland with epithelium of mesoderm-cells. 



Fig. 43. Embryo in Stage O (43). One of the germ-glands lies at the side 

 of the median line near ventral surface of embryo. 



Fig. 44. Embryo of 86 hours slightly older than Stage O. Germ-gland has 

 been carried near dorsal surface by lateral growth of embryo around the yolk. 



Fig. 45. Embryo of 105 hours (male). The germ-gland has become con- 

 stricted, forming a dumb-bell-shaped structure. 



Fig. 4G. ^Sagittal section through germ-gland (female) of an embryo 105 

 hours old, showing the developing terminal filaments, x 3-0. 



Figs. 47 to 53. Transverse sections through the tail-fold or posterior end 

 of embryos, showing the separation of the germ-cells into two germ-glands 

 which are carried near the dorsal surface by the lateral gi'owth of the embiyo 

 around the yolk. ' Fig. 47, x 1^5 ; all the others, x 140. 



Fig. 47. Embryo in Stage K (47). Part of the pole-cells are still on their 

 way through the pole-cell canal. (For sagittal section of this stage see Fig. 

 39.) 



Fig. 48. Embryo in Stage L (48). Three pole-cells on each side of the tail- 

 fold. Sagittal section in Fig. 40. 



Fig. 49. Embryo in Stage M (49). Germ-cells lie close to coelomic cavity 

 on either side of tail-fold. Sagittal section in Fig. 41. 



Fig. 50. Embryo slightly older than Stage M. Germ-cells have acquired 

 an epithelium of mesoderm-cells. 



Fig. 51. Embryo in Stage O. Germ-gland on one side of median ventral 

 line. Sagittal section in Fig. 43. 



Fig. 52. Embryo 86 hours old. Germ-gland has been carried near dorsal 

 surface. Sagittal section in Fig. 44. 



Fig. 53. Embryo 105 hours old, showing germ-gland (female) near median 

 dorsal line. The terminal filaments of the ovary are developing. 



Fig. 54. Frontal section of an embryo in Stage O. X 140. 



Fig. 55. A pole-cell and adjacent ectoderm-cell enlarged from Fig. 37 (55). 

 X850. 



Fig. 56. Two pole-cells showing amoeboid processes, enlarged from Fig. 

 38 (56). X 850. 



Fig. 57. Two pole-cells (a), two mesoderm-cells (b), and two ectoderm- 

 cells (c), enlarged from Fig. 41 (57). x 850. 



