736 PROCEEDINGS OF THE ACADEMY OF [[Oct., 



phase of next division, la just formed and lb in process of forma- 

 tion. Ic is taking in one of the polar bodies. 



Fig. 9.— Nine cells, left side. 2d just formed; Id preparing to give off Id^. 



Fig. 10. — Twelve cells, animal pole, la^ just formed. 



Fig. 11. — Thirteen to fourteen cells, animal pole, lb about to divide. 



Fig. 12.— Thirteen to fourteen cells, right side. 2c just formed; 2d=X 

 dividing to form x*. 

 Plate XLIV, Fig. 13. — Thirteen to fourteen cells, left side. 2a in process of 

 formation. 



Fig. 14.— Twenty-six cells, animal pole, lb*-' just formed; Ic'-^ and Id'-' 

 preparing to form rosette. 



Fig. 15. — Twenty-six cells from right side. Spindle fibres connecting] 3C 

 and 3c; Ic^ preparing to divide. 



Fig. 16. — Twenty-six cells, left side. 2a and 3D preparing to divide; spindle 

 fibres connecting X and x^, and 3A and 3a. 



Fig. 17. — Twenty-sLx cells, vegetal pole, x'-^ forming; 3D in metaphase 

 of division. 



Fig. 18.— Twenty-eight cells, left side. 2a' formed; cell body of 3D 

 elongated. 



Fig. 19. — Twenty-eight cells, right side. 2B in division. 



Fig. 20. — Forty cells, animal pole. Ic'-'-^ and Id'-'-^ preparing for tlieir 

 first bilateral division. 



Fig. 21. — Forty cells, right side. 

 Plate XLV, Fig. 22.— Forty cells, left side. 



Fig. 23. — Forty-two cells, right side. 4c just formed. 



Fig. 24. — Forty-two cells, vegetal pole. 4d preparing for bilateral division. 



Fig. 25. — Fifty-four cells, animal pole. Bilateral division of the posterior 

 stem cells of the first quartette completed. 



Fig. 26.— Fifty-four cells, right side. 2c^' formed. 



Fig. 27.— Fifty-four cells, left side. 



Fig. 28.— Fifty-four cells, vegetal pole. Bilateral division of 4d completed; 

 X preparing for bilateral division. 



Fig. 29. — Fifty-three cells, anterior end. Divisions of 3A, 2b, 3b, lb and 

 2c^ shown. 



Fig. 30. — Seventy-two cells. Division of cells of posterior arms of cross ; 

 arm of cross in the B quadrant just formed. 

 Plate XL VI, Fig. 31. — Seventy-two cells, right side. 



Fig. 32. — Seventy-two cells, left side. 



Fig. 33. — Seventy-two cells, vegetal pole, x^-^ divided; 5d just formed. 



Fig. 34.— Eighty-one cells, right side. 2&'^- and 2c'- divided. 



Fig. 35. — Eighty-one cells, left side. 2a}-'^ and 2s} in process of division. 



Fig. 36. — Eighty-one cells, vegetal pole. Dexiotropic division of M. and M. 



Fig. 37. — Eighty-nine cells, animal pole. Division of Ib''^-'. 



Figs. 38 and 39. — Eighty-nine cells, right and left sides respectively. 

 Plate XLVII, Fig. 40. — Eighty-nine cells, vegetal pole. 



Figs. 42-49. — Products of 2d=X viewed from the posterior end. Fig. 42 

 was drawn from the same ovum as represented in Fig. 20 ; Fig. 44 

 likewise corresponds to Fig. 30; Fig. 45 to Figs. 34-36; and Fig. 47 

 to Fig. 37. 



Fig. 50. — Vegetal pole. Formation of 5a and second division of M and M. 



Fig. 51.— Vegetal pole. 4c divided; 4a and 4b dividing. 



Fig. 52. — Division of entomeres completed. 

 Plate XLVIII, Figs. 53 and 55.— Two stages during the closure of the blasto- 

 pore. They show also the division of X and X to form x^ and x'. 



Figs. 54 and 56. — Optical sagittal sections of same, showing change in 

 form of the entoderm cells and the inward migration of their nuclei. 



Fig. 57. — Horizontal optical section of embryo at the time of the appear- 

 ance of the stomodseum. The mesomeres, which are seen in division, 

 have moved apart. In front of each mesomere is seen the band of 

 mesoblast to which it has given rise. 



Fig. 58. — Optical sagittal section of same stage, showing formation of 



