EXPLANATION OF PLATE Xll—Co7Uiwml. 



Fig. 17. — The second meridional division begins to cut tlie larger of tlie 

 two spheres, passing from the centre outwards. (5 h.) 



Fig. is. — Ten minutes later the cleavage begins on the smaller sphere, 

 advancing in the opposite direction. 



Fig. 19. — First two meridional divisions completed, (5 h. 30 min.) 



Fig. 20. — Tlie first cctoblast is in process of formation from the upper 

 pole of the largest blastomere (x). (G h. 30 min.) 



Fig. 21. — The second ectoblast in process of formation from the blasto- 

 mere (b). The first ectoblast has been pushed to the left into the blasto- 

 mere (a). (7 h.) 



Fig. 22. — Tlie third and fourth ectoblasts are forming simultaneously 

 from a and c. (7 h. 30 miii.) 



Fig. 23. — The formation of the four primary ectoblasts is completed, and 

 X begins to divide. (8 h. 30 min.) 



Fig. 21. — Thirty minutes later, from below. The division of x is well 

 advanced. (9 h.) 



Fig. 25. — The egg is seen from below. The division of x into x' and 

 X has been completed. (12 h.) 



Fig. 20. — The same from above. Three or four cells have been added to 

 the primary ectoblasts from a and b. 



Fig. 27. — Two hours later, from below, x is in process of division. 

 (14 h.) 



Fig. 2S.— Thirty minutes later. (14 li. 30 min.) 



FiG: 29. — Seen from below. The division of the original blastomere (x) 

 (fig. 22) into the two mesoblasts (x and xy), and the primary neuroblast (x'j 

 is accomplished. (11 h. 45 min.) 



Fig. 30. — Seen from above. Ten to twelve cells have been added to the 

 four ectoblasts. x' begins to divide, the plane of cleavage passing from 

 above downward. (21 h.) 



Fig. 31. — The four primary ectoblasts are completely encircled with cells 

 derived from a, b, and c. The division of x' into x- and x- is completed. 

 (24 h.) 



Fig. 32. — The same from below. 



Fig. 33. — The two cells (.?•-') are in process of cleavage. (24 h. 15 min.) 



Fig. 34. — The division of the two cells (x-) into the four cells (x^) is com- 

 plete. The four primary ectoblusls have divided each into four parts. 

 (24 h. 30 min.) 



Fig. 35. — The two median cells (x^) have pushed each other apart by 

 nipple-like protrusions of their contiguous faces. On the left, jusl above 

 (xy), are seen four cells in a row. They have been produced from the upper 

 pole of the nicsoblast (xy). (26 h. 30 min.) 



F'iG. 36. — The nipple-like protrusions have been constricted off as two 

 small median cells (x^). The mesoblast (x), which occupies the lower pole in 

 fig. 29, has changed its position, passing backward and upward, followed 

 by the ventral blastomere (c). It now lies just under the two ceils {x*). 

 This egg is seen from tiie left side. (26 h. 40 min.) 



Fig. 37. — The two cells x* have broken up into smaller cells. The two 

 median cells (r') have produced the two cells (x^), and the two outer cells 

 (x^) have produced x". This completes two synimetrically placed grou|)S of 

 neuroblasts, each group consisting of four cells {x'^, x', .r", x-*). The line of 

 mesoblastic colls produced by xy is still to lie seen. The nuclei of a, b, and 

 c have each divided into three or four parts, which are now seen in the 

 surface of the blastomeres. These free nuclei are spoken of in the text as 

 " entoplasts." The mesoblast (,r) has comjiletely disappeared, being covered 

 by the posterior ends of o, b, and c. xy has also sunk deeply into a, but is 

 still plain to lie seen. (28 h. 30 min.) 



Fig. 38. — The two neuroblasts a-* are covered with ectodermic cells. The 



