138 MALCOLM LAURIE. 



Stigmata. ^(?. Telson. ^z-. ?//e5. Trabecular mesoblast occupyiug cceloni. vil. 

 Vitreous layer of central eye. i/. c. Cells in yolk. i/k. Yolk. The somites 

 are numbered i, ii, in, &c. 



PLATE XIII. 



Fig. 1. — Transverse section of ovarian tube, showing the two layers ; one 

 cell of the inner layer enlarging to form an ovum. X ^^. 



Fig. 2. — Transverse section of ovum and ovarian tube. The egg has now 

 pushed its way through the outer layer, and appears as a small protuberance 

 on the ovarian tube. The follicle is beginning to form from the cells of the 

 inner layer, which have accompanied the ovum, x — |^. 



Fig. 3. — Longitudinal section of ovum of "1 mm. diameter, showing the 

 two-layered follicle and the yolk-forming cells (ffe/.). X ^-f^. 



F'iG. 4. — Longitudinal section of egg of "4 mm. in length, showing yolk- 

 spheres, indefinite nucleus, and strongly marked nucleolus. The egg is sur- 

 rounded by a vitelline membrane. The rest as in Fig. 3. x ^^. 



Fig. 5. — Section through the base of a ripe egg. x ^-y^. 



Fig. 6. — Yolk-spheres from ripe egg, showing the darkly stained spherical 

 and prismatic bodies and the clear spaces, x ^-y^. 



Fig. 7. — Section through a corpus luteum and part of ovarian tube. 



V 220 



PLATE XIV. 



Fig. 8. — Surface view of one-layered blastoderm, x --f-. 



Fig. 9. — Section through one-layered blastoderm, same stage as Fig. 8. 



■V 2 30 

 A ,— 1— 



Fig. 10. — Section through blastoderm later than Fig. 9, showing the cells 

 multiplying to form a mass at one pole of the egg. X - f^" 



i'lG. 11. — Section through more advanced blastoderm, x -f^. 



Fig. 12. — Transverse section through blastoderm at time of formation of 

 primitive hypoblast and serous membrane. The yolk and yolk-cells are drawn 

 in detail in this figure to show the breaking down of the former, x —f^. 



Fig. 13. — Surface view of blastoderm now becoming oval. X -^-. 



Fig. 14. — Transverse section through posterior end of embryo figured in 

 F^'ig. 13, showing serous membrane, primitive thickening, primitive hypoblast, 

 and " peripheral cells." x ^^-. 



Fig. 15. — Transverse section through anterior part of embryo about the 

 same stage, x -f-. 



Fig. 16. — Longitudinal section through an embryo a little younger than 

 i'ig. 17, showing two somites with a third forming. The mesoblast is 

 forming from the primitive hypoblasi, the amnion is growing up fium the 



