Chap. XVI.] THE FRESH-WATER MUSSEL. 



319 



clear and glassy (4). In such ova which may be obtained in 

 quantity in December by making a clean incision in the upper 

 part of the foot of a recently killed mussel, and mounting the 



FIG. 94. MUSSEL: OVA, SPIRAL STARS, GLOcmriA. 



1, 2, 3. Young ova torn from the ovary. 4. Ripe ovum dehisced. 5. Ovum, 

 showing spiral star perhaps accompanying the formation of the polar cell. 

 6. Spiral star under high power (Gundlach's ^ inch immersion). 7. Glochi- 

 dium with byssus and tactile organs. 8, 9. Glochidia shells. 10, 11. Beak 

 of Glochidium under high power. 



fluid which exudes beautiful spiral stars may be seen (5 and 6). 

 One such star rarely a second may be seen in each ovum. 

 They perhaps accompany the formation of the polar body. 



Development. The ova undergo the early stages of their 

 development in the external gills of the mother. Here, in all 

 probability, they are fertilised by spermatozoa shed into the 

 surrounding water by a male anodon, and carried inwards by 

 the inhalent current. 



Segmentation is unequal. The granular protoplasm of the 

 impregnated ovum buds off in succession a number of small clear 

 cells. These small cells are free from yolk, the mother cell 

 retaining all the food-yolk. Between this large yolk-segment 

 and the small clear cells a cavity the segmentation cavity is 

 formed. From the small clear segments epiblast will be formed. 



