GAR-PIKE AND STURGEON. 6 1 



EXPLANATION OF PLATE IV. 



The Early Development of Acipenser stiirio. 



Fig. 55. Vertical section of egg immediately before first cleavage. X 22. 



Fig. 56. Horizontal section (slightly oblique) of 4-cell stage, through the 

 region of the nuclei. X 22. 



Fig. 57. Horizontal section of S-cell stage, through the region of the nuclei. 

 An irregular cleavage is to be seen in the left side of the figure : the first cleavage 

 plane of this and of the preceding figure passes right and left. X 25. 



Fig. 58. Vertical section of 5-hour stage, showing cell outlines in the floor of 

 the segmentation cavity. X 25. 



Fig. 59. Vertical section of a stage of nearly the same age as that of Fig. 58, 

 showing in greater detail the relation of the yolk cells to the mode of growth of 

 the blastoderm. X about 65. 



Fig. 60. Vertical section of 6-hour stage. X 25. 



Fig. 61. Vertical (sagittal) section of stage, immediately prior to gastrulation. 

 The posterior region of the embryo is at the right of the figure. X 25. 



Fig. 62. Sagittal section of early gastrula (17 hours). The region of the dor- 

 sal lip is notably pigmented, and the outlines of the yolk cells in its vicinity may 

 now be outlined. X 25. 



Fig. 63. Sagittal section of gastrula of 26 hours, in which the ventral lip of 

 the blastopore appears as but an indentation at the right side of the figure. The 

 middle, inner, and outer germ layers are seen to be present in the dorsal lip. 

 X 25. 



Fig. 64. Vertical section near sagittal plane of gastrula of 29 hours. The 

 inner, middle, and outer germ layers of the dorsal lip are lettered, /, m, o ; the 

 thinness of the inner layer is here accounted for in its proximity to the chorda ; 

 segmentation cavity is lettered s\ coelenteron, c ; and the vesicle of Kupffer, /■. 



X 25. 



Fig. 65. Section near the sagittal plane of region of the blastopore in stage 

 of 32 hours. The structures of the dorsal lip are lettered as in preceding figure, 

 and are seen readily comparable with those of the ventral lip. X about 65. 



Fig. 66. Vertical (nearly sagittal) section of embryo of 43 hours. The thick- 

 ening of the outer layer in the head region is lettered k, and although the yolk 

 plug is now greatly constricted, the structures of dorsal and ventral lips and the 

 enlarged coelenteron may readily be made out by comparison with Fig. 65. As 

 yet the Deckschicht is not clearly defined and has been outlined in the figure. 



Fig. 67. Transverse (slightly oblique) section through the region of the neu- 

 renteric canal of an embryo somewhat older than that of Fig. 66. Germ layers 

 are indicated as before : neurenteric canal, n ; yolk cells, y ; yolk plug, yp, now 

 disappearing; on either side of its position is the undifferentiated cell mass. 



X4S- 



Fig. 68. Section almost sagittal of neurenteric canal of embryo of 46 hours. 

 It is now seen definitely established, n ; the site of the yolk plug is indicated as a 

 slight diverticulum of its hinder wall, jj//. X about 100. 



Fig. 69. Sagittal section of neurenteric canal of embryo of 58 hours. It now 

 opens widely into the gut, ;/, and its cellular wall is well established ; the caudal 



