SEA MUSSEL MYTILUS EDULIS. 1 85 



cytoplasm. As fast as the ripe ova are formed they burst out into the follicles and 

 canals which they come to fill so tightly that they are compressed into a characteristic, 

 polygonal form. A cross section of either the mantle or mesosoma at the height of the 

 breeding season will show how completely these organs are filled and distended with 

 eggs (fig. 215,0pp. p. 226). 



The sperm follicles just previous to the reproductive period, when seen in cross 

 section, are irregularly circular in outline, from 300 to 800 microns in diameter. They 

 are almost completely filled with sperm mother cells and spermatozoa, the latter occupy- 

 ing the central portion. The peripheral region of the follicle is occupied with large 

 sperm mother cells, which consist of nuclei surrounded with only a film of cytoplasm and 

 containing one or two large nucleoli. Passing toward the center of the cavity, the nuclei 

 become smaller and the contained chromatin is in the form of threads, which indicates 

 that the cells are in the active process of division. In no preparations, however, was a 

 single mitotic figure observed. In the center of the follicle spermatids and spermatozoa 

 were present which were very small compared with the mother cells and which stained 

 uniformly deep blue or black with iron hematoxylin (fig. 176, p. 184). Later in the season 

 when the ripe products are ready to be liberated, the follicles present a different appear- 

 ance. They are densely filled with spermatozoa, which appear as minute, round dots 

 arranged in bands or lamellae which converge toward the center, or they may be so 

 arranged as to make a coarse network (fig. 220, opp. p. 227). 



The number of follicles which are contained in the mantle of a mussel depends upon 

 the age and size of the animal. In small specimens just approaching maturity a single 

 follicle will fill the space between the outer and inner walls of the mantle, while in large 

 specimens, 3 or more inches long, the same space may accommodate a series of 6 to 8 

 follicles (fig. 220). 



physiology. 



When the genital products are first formed, they are mature to all appearances, 

 morphologically, but physiologically they are immature, for when such eggs and sper- 

 matozoa are mixed together fertilization fails to take place. The spermatozoa have 

 tails but make no movement, and the eggs, though containing a well-developed germina- 

 tive vesicle, fail to form the spindle of the first polar body when they come in contact 

 with the sea water as is normal for mature ova. Both elements are perfectly inert. 

 Before reaching functional activity they must undergo a period of rest which apparently 

 lasts for several weeks. As they reach the stage of functional activity they begin to 

 crowd out into the main canals and are swept onward by strokes of the powerful cilia 

 which line one side of the canals (fig. 177, CEp). They are forced up close to the genital 

 opening, which is closed by two lips of tissue and furthermore sealed by a plug of granules 

 and minute pigmented cells which are reddish brown in color. 



The expulsion of the reproductive elements begins suddenly and takes place rapidly. 

 The first sign is when the plug of pigmented granules is discharged. This is followed, in 

 the male, by a continuous stream of milt which renders the water milky for a distance 

 of several feet from the spawning individual. The discharge may continue for half an 

 hour, with little or no interruption. In the female the process is similar, except that 

 the eggs usually are expelled while sticking together in the form of rods, which may be 

 from 3 to 5 mm. in length. They represent the form into which they were molded 

 together while in the oviduct. The eggs flow out in a continuous stream and in quiet 



