300 



A. IZUKA. 



Sexual Products. — The eggs or the spermatozoa are discharged, 

 while the mature worms are actively swimming near the surface of water. 



The eggs sink down gradually to the bottom. They are each sur- 

 rounded by a thick and transparent gelatinous envelope, so that when 

 found in masses, they are separated from one another by a considerable 

 space. They are spherical in shape, with a diameter of 140-160 p. In 

 colour they vary from greenish yellow to deep green. 



The spermatozoa are found adhering in large numbers to the gel- 

 atinous envelope of the ovum. Each spermatozoon consists of an 

 ellipsoidal head and of a long filiform tail. The former is 3.2 /x. long and 

 1.8 [J., broad. The latter measures 35-40 fi. in length ; it gradually tapers 

 toward the hind end. 



2. On the Fertilization of the Mßggs and 

 their Development. 



Artificial fertilization by bringing together the eggs and spermatozoa 

 taken from mature worms can easily be effected, provided with precaution 

 be taken to keep the water at a constant temperature (about 1 5°c) and its 

 salinity the same as at high tide in Kojima Gulf. 



About ninety minutes after fertilization, the first polar body is ex- 

 truded ; the second follows about thirty minutes later. 



The cleavage of the egg is total and unequal. The first cleavage 

 takes place about three hours after fertilization (text-figure 2, A.) ; the 

 second is accomplished about fourty minutes later (text-figure 2, B); the 

 third is completed (text-figure 2, C) about five hours and a half after 

 fertilization. The cleavage process agrees in general with that described 

 by E. B. Wilson* for Nereis limbata ; only it seems to proceeds much 

 more slowly. 



Ciliated embryos first appear about 40 hours after fertilization ; 4 or 5 

 hours after that the embryos are seen swimming about very actively in the 

 vessel containing them. They then show a marked positive heliotactic 



* Wilson, E. B.— The Cell-liniage of Nereis. Jour, of Morphology Vol. VI. 1892. 



