BIGELOW. Development of Lepas. 



PLATE 7. 



Figures drawn from transparent preparations of entire eggs. Vegetative pole 

 and blastopore at the right side in figures seen in lateral view. 



Fig. 56. Optical section in sagittal plane. Sixty-two cells, counting the dividing 

 primary mesoblasts (c? 6-3 , t/ 6 **) as four cells. 



Fig. 57. Same stage. Actual section. Blastopore not completely closed. 



Fig. 58. View from vegetative pole. The mes-ectoblasts (a, 6 6 - 3 , 6 6 ' 4 , c) in 

 sixth cleavage, which results in forming the " secondary mesoblasts." 

 Blastopore slightly open. 



Fig. 59. Same egg in optical section in parasagittal plane. The primary meso- 

 blasts (rf 6 ' 3 , c? 6>4 ) not yet in sixth cleavage. Two entoblastic nuclei 

 (c? 6 -i, rf 6 - 2 ). Mes-ectoblast cells 6 6 ' 3 and c 6 ' 3 dividing parallel to the 

 surface of blastoderm, to form " secondary mesoblasts." 



Fig. 60. View from vegetative pole of egg in which the primary mesoblasts 

 (c? 6<3 , ft-*) have not been overgrown by the blastoderm during the sixth 

 cleavage. These cells nearly fill the blastopore ; the posterior pair of 

 "secondary mesoblasts" (a?- 5 , c 7 ' 5 ) lie at the sides of the primary 

 mesoblasts. 



Fig. 61. Optical section near sagittal plane of same egg, showing anterior pair of 

 " secondary mesoblasts" (6 7 - 5 and b 1 ' 1 ) and two entoblast nuclei. 



Fig. 62. View from vegetative pole of egg with fifty-six blastoderm cells, four 

 " secondary mesoblasts " (a 7 ' 5 , 6 7 * 7 , 6 7>5 , c 7 ' 5 , represented by broken 

 lines), two dividing primary mesoblasts (rf 6 ' 3 , c? 6 ' 4 , outlines shown by 

 fine continuous line), and two entoblast nuclei (seen at deeper level 

 but not figured). 



Figs. 63, 64. Optical sections in horizontal plane of different eggs, viewed from 

 vegetative pole. Same stage as Figure 56. Figure 63 represents a 

 common condition in which mesoblasts and entoblasts are not separated 

 by the sagittal plane. 



Fig. 65. Optical section in sagittal plane of egg with sixty-two cells. The 

 primary mesoblasts have completed the sixth cleavage, forming c 7>5 - 8 . 



