DESCIUPTIOX OF FIGURES 



All figures represent entire eggs of Crepidula plana, fixed, stained, and mounted 

 on slides in balsam. They were drawn by means of a camera lucida with Zeiss 

 apochromatic oil immersion Obj. 3 mm., Ocular 4, at stage level and are thei-e- 

 fore magnified 333 diameters. In the process of reproduction they have been 

 reduced one-third. 



The oil droplets, indicated by a coarse aveolar structure, mark the centrip- 

 etal pole; the great mass of yolk, which is left unshaded, lies in the centrif- 

 ugal half of the egg; while the cytoplasm of the middle zone is shaded b\' 

 stipples, the more granular part (spongioplasm) being stippled more densely 

 than the less granular part (hyaloplasm) when these two are separated. The 

 eggs drawn usually represent common types of abnormalities produced by cen- 

 trifuging. Thousands of other kinds of abnormalities are produced, indeed no 

 two are ever identically the same. 



PLATE 1 



EXPLAN.\TION OF FIGURES 



1 and 2 (1126, 1) Eggs taken while being laid and centrifuged in the ger- 

 minal vesicle stage for 10 minutes; fixed immediately after centrifuging. The 

 segregation of yolk and cytoplasm is fairly complete. The eggs are flattened 

 in the axis of centrifuging and the nuclei are elongated in that axis; the chromo- 

 somes and centrosomes are at the heavier end of the nucleus, the nucleolus is at 

 the lighter end. The grouping of the chromosomes shows that the axis of the 

 future spindle will be oblique to the long axis of the nucleus in figure 1, trans- 

 verse to it in figure 2. 



3 and 4 (1125, 1) Centrifuged 10 minutes (3000 revolutions per minute), 

 fixed at once. The nucleolus is thrown with the fatty substance to the lighter 

 pole of the egg; the sperm nucleus remains in the yolk in figure 3, in the cyto- 

 plasm in figure 4. The two poles of the spindle are equidistant from the cell 

 surface and are not in contact with it. Figures 3, 4, 7, 8, are from the same slide, 

 figures 9 to 12 are from the same experiment Ih hours after centrifuging. Eggs 

 of this same lot 24 hours after centrifuging show both polar bodies in the middle 

 of the ectodermal plate and everything absolutely normal. 



5 and 6 (1038) Centrifuged 10 minutes, fixed 3 hours later. The cyto- 

 plasm has been moved to the vegetative pole. The first maturation spindle has 

 remained attached to the animal pole and its traction on the cell membrane 

 is shown by the indentation of the latter. The sperm nucleus lies near the fatty 

 substance of the lighter pole of the egg in figure 5, and in the yolk in figure 6. 

 In figures 5 and 6 the spindle was attached to the cell membrane at the animal 

 pole before centrifuging began. 



374 



