EXPLANATION OF THE PLATES. 



PLATE XX. 



Fig. 1. A mature egg just fertilized. Numerous round refractive granules, evenly distributed through 

 the cortical layer, render the egg somewhat opaque. The cortex of uniform thickness, no 

 disc being recognizable. X 50. 



Fig. 2. A clear area with granular border at the centre of the blastodisc, in which the liist cleavage-nucleus 

 was seen two minutes earlier. Cleavage followed in twelve minutes. X 50. 



Fig. 3. Five minutes after fertilization. A clear area with a nucleus (first polar spindle ?) seen at. centre 

 of the disc, which is just becoming visible as a shadowy ring. The first cleavage groove in 

 this egg became recognizable 3D minutes later. Examined in profile, the disc bad a conical 

 shape, with its rounded summit looking towards the centre, of the egg. The tip of the cone 

 was clear, while the rest of the disc was pale straw-colored, like the periblasts envelope. The 

 nucleus was not seen in I he profile view. X 50. 



Figs, 4 </, It, c. The pronuclei in process of conjugal ion. A clear space, shown by the dotted line, sur- 

 rounds them. Conjugation completed in eleven minutes, and one minute later the cleavage 

 nucleus (! became invisible, entering upon the arnphiastral stage. X 280. 



Fig. 5. The cleavage begins by an elongated indentation at the centre of the inner surface of the disc. 

 The disc has a shadowy oval outline. About 35 minutes alter fertilization. X 50. 



Figs. 5-29, and 34. Successive stages as seen in the living egg, followed from 10 h. 1? in \. \i. to 5 b. 

 30 m. r. m., July 5, 1883. X 50. 



The ages of these stages, which extend over only 6 hours and 13 minutes, are as follows : — 



Fig. 5 = 35 minutes. Fig. 14 = 1 hour 8 minutes Fig. 23 = 2 hours 1 minute. 



" 6 = 37 " " 15 = 1 " 12 " " 24 = 2 " 18 " 



" 7 = 40 " " 16 = 1 •' 15 " " 25 = 3 " " 



" 8 = 43 " " 17 = 1 " 18 " " 26 = 3 " 35 " 



" 9 = 48 " " 18 = 1 " 25 " " 27 = 4 " 3 • 



" 10 = 51 " " 19 = 1 " 29 " " 28 = 4 " 48 " 



" 11 = 55 " " 20=1 " 35 " " 29 = 6 " 48 " 



" 12 = 57 '• " 21 = 1 " 40 " " 30 = 7 " 18 " 

 " 13 = 60 " " 22= 1 " 53 " 



Figs. 311. 31. An irregular S-cell and a 16-eell stage, both from same egg, twenty minutes apart. Developed 



in the live-box into a perfect embryo. X 50. 

 Figs. 32, 33. Another example of the 8-ecll and 10-ccll stage, from an egg that developed a normal 



embryo. X 50. 

 Fig. 35. An S-cell stage found July 4, 18S3. These eight cells next divided concentrically into sixteen, 

 then radially into thirty-two. The egg was nol followed further. This form has been 

 noticed twice. X 50. 

 A peculiar 16-eell st;iu r e. Species undetermined. X 50. 

 A 16-eell stage of Ctcnolabrus, after treatment with osmic aeid .ami MerLrl's fluid, showing a 



marked contrast between central and peripheral cells. X 50. 

 Egg of the Tautog, in which the lirst two blastomeres assumed a perfectly circular outline, aud 



were separate. X 50. 

 Another egg of same species, showing a peculiar l-cell stage. X 50. 



