54 PROTOPLASMIC CONSISTENCY AND CELL DIVISION 



The mature starfish egg averages 0.16 mm. {i.e. 160 /x) in diameter. 

 The needles used for dissection averaged 10 ix in thickness at about 

 1 mm. from the tip and tapered gradually from there to a point far 

 below 1 II. With such a needle one can make a puncture or a clean 

 cut through the egg in any desired spot or plane without causing 

 apparent disturbance in the protoplasm of the egg. For cutting 

 purposes glass needles as shown in Fig. 2 were used.^ As the egg 

 lies suspended in a hanging drop the end limb of the needle (Fig. 2 a) is 

 set in such a way as to push the egg against the cover-sHp. Con- 

 striction of the egg is produced by a continued upward pressure of 

 the needle until the egg is cut in two. The operation does not neces- 



C overs] (proofing' moisi: chamber 

 Ei^^inhon^in^di'pp 



End limbo) needle^ 



a b 



Fig. 2. Methods used for cutting an egg in two. a, side view of moist chamber 

 magnified to show needle in position with its end limb so placed as to compress 

 an egg between it and the cover-slip. Continued pressure of the needle cuts the 

 egg in two. b, a second method of cutting an egg by bringing the end limb of 

 the needle down on the egg so as to press the egg against the lower surface of the 

 hanging drop. 



sarily destroy the fertihzation membrane which envelops the egg. 

 The egg may also be cut in two on bringing it (Fig. 2 b) between the 

 end limb of the needle and the lower surface of the hanging drop. 

 Lowering the needle out of the drop in such a way as to give to the 

 egg a rolHng motion cuts the egg cleanly in two. This second method 

 is not as satisfactory as the first for cases where one wishes to preserve 

 the spatial relations of the egg contents, as the rolUng motion pro- 

 duces churning movements within the cell. 



Experiment 2. — (Figs. 3 to 7.) An Asterias ovum just beginning 

 to segment and with the amphiaster in full development was cut 



^ Chambers, R., The microvivisection method, Biol. Bull., 1918, xxxiv, 121. 



