144 BIOLOGICAL LECTURES. 



the surface in actual contact with the membrane is correspond- 

 ingly very great as compared with small eggs with ample space 

 between the yolk and the membrane. The conditions, then, 

 are most favorable for strong adhesion. 



Born asserts that " such an adhesion cannot hold for more 

 than the first stages." As this statement does not rest upon ob- 

 servation of the toadfish ^2,%, it can have no value beyond that 

 of conjecture suggested by experience with other eggs. It is 

 quite true that six days gives ample time for any number of 

 rotations, but it is not a question whether there is time for rota- 

 tion, but zvJiether rotation actually occ7irs, and that can only be 

 determined by close study of the developing ^%^, not by sec- 

 tioning of hardened and imbedded eggs, nor by any amount of 

 experiments on frogs' eggs. 



In one very important respect the toadfish Qg^ is far superior 

 to the frog's o.^^ for the study of the question here considered, 

 since it can be observed under perfectly normal conditions, 

 without resort to those artificial means of fixation or marking 

 which are necessary in a frog's Qg,%, and which must always 

 cast some doubt on the reliability of the results. 



In the experiment of '91 the method of determining the 

 relation of the first cleavage plane to the axis of the embryo 

 was as follows : After the eggs had become fixed by the adhe- 

 sive disk to the bottom of a glass dish they were artificially 

 fertilized. The blastodisk appeared on the free pole of the 

 ^gg, where it was easily watched by means of a lens. The 

 eggs were plotted on paper, each &gg being represented by a 

 circle (Fig. Ill), and the paper and the dish containing the eggs 

 oriented by fastening a label on each in the same relative posi- 

 tion. When the first line of cleavage appeared the direction 

 was indicated in the circle representing the ^gg, by the diame- 

 ter, and when the axis of the embryo became visible, that was 

 indicated by an arrow drawn across the same circle. 



The first furrow appeared seven hours after fertilization, and 

 on the seventh day the axis of the embryo could be distinctly 

 seen as a light streak in the blastoderm. 



The result of this experiment is seen by an examination of 

 the circles given in Fig. III. Of the twenty-three developing 



