456 HOLMES, 



EXPLANATION OF PLATE XX. 



Fig. 37. Apical view of an egg of about 150 cells. 2<5'-' is in process of divi- 

 sion, although this change had taken place in the earlier stage shown in Fig. 31. 

 A longitudinal splitting has taken place at the base of the lateral arms of the 

 cross. 



Fig. 38. Vegetal pole of the same egg. The lower side is flattened and 

 slightly depressed in the center. In the b quadrant two of the secondary meso- 

 blast cells have lost connection with the surface, and two others, 3^2.2.1.2 ^^^ 

 2,52.1.1.2^ have still a small portion of their surface visible at the outside of the 



egg- 



Fig. 39. Anterior view of an egg of about the same stage as the preceding. 

 The tip cells of the anterior arm of the cross have become clear, and are being 

 pushed by each other by the forward rotation of the apical pole. The lower cells 

 of the second quartette are somewhat dislocated by the same process. 



Fig. 40. Apical view of a later stage, showing the two upper cells of the 

 second quartette of the anterior side of the egg now lying side by side. 



Fig. 41. Vegetal pole of an egg of about the same stage. The depression 

 of the entoderm is deeper than in Fig. 38. A division has apparently occurred 

 in 23^-^-' and in ih^-^-^. The middle cells of the anterior quadrant of the second 

 quartette, 2(^^-'-', 23^-'-^ 2(^*-^-', and zb^-^-^, have been dislocated by the rotation of 

 the apical pole so that they lie in a transverse line. The secondary mesoblast 

 cells no longer appear at the surface of the egg. The stomatoblasts in the a, b, 

 and c quadrants still remain undivided, while the corresponding cell of the d quad- 

 rant is crowded away from the entomeres by the cells of the third quartette, which 

 come to meet in the middle line {cf. Figs. 30, 36, 38, and 41). 



Fig. 42. Apical pole of a later stage. The anterior rotation of the apical 

 pole is increased, and the cells of the middle of the cross are beginning to enlarge 

 and become clear. 



Fig. 43. Right side of same egg. 



Fig. 44. Lower pole of same egg ; mouth of gastrula reduced to a slit. 



Fig. 45. Posterior side of a gastrula, showing the large head vesicle. 



Fig. 46. Ventral side of the same gastrula, showing the blastopore reduced 

 to a minute slit lying between a pair of oblong cells. Prototroch shown by a band 

 of clear cells in front of the blastopore. Rudiments of the cerebral ganglia shown 

 by two patches of dark cells separated by a median band of large, clear cells, the 

 apical plate A. P. 



Fig. 47. A later stage, showing the prototroch, apical plate, A.P., blastopore, 

 bl., and the rudiment of the right cerebral ganglion. The cell boundaries shown 

 in the figures in this plate are not diagrammatic, but represent accurately the out- 

 lines of every cell. In eggs stained with silver nitrate these boundaries are as 

 clear as shown in the figures. 



Fig. 48. View of anterior portion of a gastrula, showing the cell i3'-^' ' 

 pushed forward until it comes in contact with the prototroch. 



