126 



OHAS. B. WILSON. 



rounded by a zona pellucida^ which swells up in fifteen or 

 twenty minutes to about twice the diameter of the egg itself 

 (fig. 20). 



This zona pellucida is entirely distinct from the gelati- 

 nous envelope already mentioned, from which it is separated 

 by two concentric membranes, situated close to each other 

 and at quite a distance from the surface of the egg. The 

 membranes and the zona pellucida itself are perfectly colour- 

 less and transparent. Soon after the eggs are placed in 

 water for artificial fertilisation; or soon after they enter the 

 water when laid naturally, the outer gelatinous envelope 

 dissolves and disappears, leaving the egg surrounded only by 

 the zona pellucida and the membranes. If the eggs are 

 placed in water before they are fully ripe the outer envelope 

 does not disappear, but remains holding the eggs together in 

 bunches. 



After the ovum is pinched off into the central cavity of the 

 ovary it retains its original flask shape for a long time, some- 

 times even after it is laid. Hence it occasionally happens 

 that after the outer gelatinous envelope has disappeared the 

 limiting membranes will be found to possess a teat-like pro- 

 tuberance on one side, which is manifestly the remains of the 

 original connecting stalk (fig. 20). And sometimes a cor- 

 responding remnant may be found upon the egg itself, the 

 neck of the old flask which has not been wholly withdrawn 

 into its body. These protuberances are of special signifi- 

 cance, because they enable us to orient the eggs perfectly, 

 and to determine that the polar bodies always come off at a 

 point diametrically opposite to them, and hence opposite to 

 the original point of attachment. It should be added that 

 since the egg is always elongated into its flask shape at 

 approximately right angles to the surface of th e epithelium, 

 it follows that the egg axis bears no definite relation to the 

 axis of the mother, but may stand at any angle with it. 



Spermatogenesis. — The testes are formed and developed 

 in the same way as the ovaries. The origin of the spermatozoa 

 in Lineus has been admirably worked out by Lee (30). He 



