546 



THE MICROSCOPE. 



out the difterent parts of the thread. The Limnoeu8 stag- 

 nalis deposits small sacs, containing from fifty to sixty 



Fig. 252. — LimneBa stagnalis. 



0va; one of -which is represented at a, fig. 252. When 

 examined soon after they are deposited, the vesicles appear 

 to be filled with a perfectly clear fluid; at the end of 

 twenty-four hours a very minute yellow spot, the nucleus, 

 or germ, may be seen near the side of the cell-wall. In 

 about forty- eight hours afterwards, this small germ has a 

 smaller central spot rather deeper in colour, which is the 

 nucleolus. On the fourth day the nucleus has changed its 

 position, and is enlarged to double the size : a magnified 

 view is given at h ; upon viewing it more closely, a trans- 

 verse fissure or depression is seen ; this on the eighth day 

 most distinctly divides the small mass into the shell and 

 soft part of the future animal, c. It is then detached from 

 the side of the cell, and moves with a rotatory motion 

 around the cell-interior; the direction of this motion is 

 from the right to the left, and is always increased when 

 the sunlight falls upon it. Tlie increase is gradual up to 

 the sixteenth day, when the spiral axis can now be made 

 out as at d; it presents a striking difference in appearance 

 to the soft parts. On the eighteenth day, these changes 

 are more distinctly visible, and the ova crowd down to the 

 mouth of the ova-sac ; by using a higher magnifying power, 

 a minute black speck, the future eye, is seen protruded 



