SEA MUSSEL MYTILUS EDULIS. 1 89 



MATURATION AND FERTILIZATION. 



That fertilization of the mussel egg takes place in the water has been observed by 

 Mcintosh (1885) and Wilson (1887), while Scott (1901), who studied mussels kept in 

 tanks, believes that impregnation of the eggs occurs in the branchial chamber of the 

 mother. According to his observations, "the embryos flow from the female in a slow 

 distinct stream." Of the hundreds of spawning mussels which have been under the 

 author's observation, not one to his knowledge has discharged fertilized eggs into the 

 water. The eggs are discharged in short rodlike masses, which have been described in 

 the chapter dealing with the reproductive system. If the water is quiet they settle to 

 the bottom and the eggs fall apart, forming a brick-red to pinkish mass, according to the 

 amount of pigment present, which is variable. If the locality is subject to the influence 

 of currents or wave action, the eggs flow away and are more or less widely scattered 

 over the bottom. Spermatozoa are generally liberated into the water at the same time 

 that eggs are being shed, and with their long, rapidly vibrating tails, they are able to 

 locomote with surprising rapidity. They are attracted to the eggs, about which they 

 cluster in large numbers, with their conical pointed heads pressing against the vitelline 

 membrane and the tails beating so that the egg keeps twisting around with a spiral 

 motion (fig. i8ie). Normal freshly laid eggs permit but one spermatozoon to enter, 

 whereas stale or anesthetized eggs allow many to penetrate the cell wall. 



As soon as the spermatozoon enters the egg, rapid changes begin to occur in both 

 bodies. The spermatozoon loses its tail and the head begins to increase greatly in size 

 as it moves toward the center of the egg, leaving a clear path behind it (fig. 1 79, Sp, p. 184). 

 The spindle of the first polar body, which has remained stationary at the end of the pro- 

 phase period since the egg was first deposited, now becomes active again. The chromo- 

 somes in the equatorial region divide and migrate toward their respective poles (fig. 179), 

 and at the same time the egg, which hitherto has had no definite form, becomes a regular 

 sphere. When the eggs assume the spherical form it is a pretty sure sign that impreg- 

 nation has occurred. 



The first polar body appears in from 18 to 20 minutes after the eggs and spermatozoa 

 are mixed together, provided the sperms are in an active state (fig. 181/, p. 188). Wilson 

 (1887). states that the polar cells appear four hours after mixing ova and sperm, which 

 indicates that his material was abnormal or that the water was very cold. For several 

 years the author has been repeating the experiment with the uniform result that in sea 

 water at about 68° F. the first polar cell appears within 20 minutes after the eggs are 

 discharged and mixed with spermatozoa. 



Ten minutes after the appearance of the polar cell a second polar cell is extruded 

 behind the first (fig. 181, g and h, p. 188), after which the egg remains quiescent, to all 

 external appearances, for a period of about 20 minutes. 



At the end of this period the vitelline membrane on the side of the egg opposite to 

 the polar bodies becomes wrinkled, and the margin of the cytoplasm adjacent to it takes 

 on an irregular and wavy outline (fig. 181, h and i, p. 188). This end of the egg, which 

 represents the vegetative or nutritive pole, now begins to protrude itself in sucfi a way 

 as to give the egg a pear shape, the stalk of the pear including the nutritive pole and the 

 broad end the formative pole. At the same time, two clear spots are usually visible in 

 the granular cytoplasm, which represent the male and female pronuclei (fig. 181;", p. 188). 

 90392°— 22 5 



