28 ART. 11. — A. IZUKA : OBSERVATIONS ON THE 



from twenty to thirty are present in each egg, vary much in size 

 and are situated in the vegetative half of the vitelline mass. This 

 pole of the egg is therefore lighter than the opposite and is thus 

 always turned upwards in the natural position of the egg in 

 water. The second kind of oil-drops are minute and highly 

 refractive spheres, found scattered throughout the entire vitellus. 

 I am unable to say whether they are chemically of exactly the 

 same nature as the larger kind. There exist no intermediate sizes 

 linking together these two kinds of drops. The deutoplasmic 

 spheres may be said to stand in point of size intermediate between 

 the two forms of the oil-drops. They are at first uniformly dis- 

 tributed in the vitellus together with the smaller oil-drops. 



The spermatozoa, soon after expulsion from the body, are 

 found adhering in large numbers to the gelatinous envelope of 

 the ovum. Each consists of an ellipsoidal head and of a long- 

 slender filiform tail. The head is 3^ long and 1.7/^ broad. The 

 tail measures 35-45 p- in length ; it gradually tapers towards the 

 hind end. 



Artificial fertilization by bringing together the eggs and 

 spermatozoa taken from mature worms can easily be effected, 

 provided the precaution be taken to keep the water at the same 

 temperature and the same degree of salinity as that at high tide 

 in the river during the swarming period. Forty to fifty minutes 

 after fertilization, the vitellus contracts and aquires an irregular 

 surface, which is separated here and there from the vitelline 

 membrane by vacant spaces. Sometime afterwards, the vitellus 

 again assumes a perfectly spherical shape, and is then separated 

 throughout from the membrane by a narrow peri vitelline space. 



Meanwhile certain changes, preparatory to the formation of 

 polar bodies, take place in the animal pole. The deutoplasmic 



