MICRODISSECTION STUDIES. 327 



the pressure divides the egg into two pieces. In the sea-urchin 

 egg the two pieces immediately round up and roll away from one 

 another. In the starfish egg the tougher membrane is not rup- 

 tured, but holds the two pieces together. 



The membrane of the sea-urchin egg is so delicate that it is also 

 possible to cut the egg in two in the following manner : In a hang- 

 ing drop the horizontal end of the needle is brought over the egg 

 (Fig. 10). The needle is now lowered. This brings the needle 



Fio. 10 



o 



FIG. jo. Side view of moist chamber to show one method of cutting an 

 egg in two with the microdissection needle. 



against the upper surface of the egg and presses the egg down 

 against the surface film of the hanging drop. On lowering the 

 needle still further it passes through the egg and out of the drop, 

 cutting the egg cleanly in two. In the case of the starfish egg this 

 procedure would drag the egg out of the drop along with the 

 needle. The membrane of the sand-dollar egg is weaker than that 

 of the starfish and stronger than that of the sea-urchin egg. 



The consistency of the membrane varies with the age of the 

 egg. The full-grown immature egg of the starfish has a relatively 

 tough membrane. On the other hand, young ovarian eggs possess 

 very delicate membranes and they can be cut in two with the same 

 ease as mature sea-urchin eggs. 



The strongest argument regarding the existence of a mem- 

 brane about the unfertilized egg is that a membrane may be 

 stripped off the egg whereupon the egg, which was previously 

 non-adherent, now sticks to everything it touches. The fer- 

 tilizability of such naked eggs is discussed under the next head- 

 ing. 



The existence of egg membranes is a fairly universal feature and 

 it is, therefore, not surprising that we should find them in the 



