Ije WILSON. [Vol. VI. 



scattered at rather wide intervals through the entire vitellus. I 

 am unable to say whether they are of the same nature as the 

 large drops, but intermediate forms seem not to exist. The 

 deutoplasm-spheres (Fig. i) are intermediate in size between 

 the two forms of oil-drops. They are at first equally distrib- 

 uted through the vitellus, and only after fertilization do they 

 begin to disappear from the upper pole (as will be described 

 further on). In appearance they are clear and homogeneous, 

 approximately spherical, but with rather irregular outlines, more 

 highly refractive than the protoplasm, but much less so than 

 the oil-drops. 



When first discharged, the eggs are somewhat irregular in 

 form. Soon, however, they become perfectly spherical with a 

 diameter of about 0.12 to 0.14 mm. in N. limbata, considerably 

 less in N. inegalops (the eggs of both species vary considerably 

 in size). The vitellus is at first surrounded by two membranes, 

 the outer of which is extremely thin and delicate, the inner 

 (zona radiata) much thicker and with very distinct radial stria- 

 tions. I shall not attempt in this place to give any account of 

 the internal phenomena of fertilization, but there are certain 

 peculiar external phenomena to which it may be useful to call 

 attention. From twenty to thirty minutes after fertilization 

 the striae of the zona suddenly become indistinct, and in the 

 course of two or three minutes the zona itself entirely disap- 

 pears, leaving only the outer membrane. The vitellus mean- 

 while becomes irregular in form, and after the disappearance 

 of the zona, assumes an amoeboid appearance, so as to be here 

 and there widely separated from the membrane. For about ten 

 minutes slow amoeboid changes of form take place, after which 

 the vitellus again becomes perfectly spherical, but is separated 

 from the outer membrane by a slight space. Meanwhile cer- 

 tain changes, preparatory to the extrusion of the polar cells, 

 take place at the upper pole. 



The freshly laid ^gg has a large germinal vesicle which lies 

 excentrically, somewhat above the centre of the vitellus ; a few 

 minutes after fertilization the vesicle disappears. Towards the 

 close of the amoeboid phase the deutoplasm-spheres begin to 

 migrate away from the upper pole, leaving a clear polar area 

 composed of granular protoplasm. In it may be seen a few 

 scattered oil-drops of the small form, but deutoplasm-spheres 



