SEA MUSSEL MYTILUS EDULIS. 1 87 



The number of eggs spawned by a female mussel at a given time can be determined 

 fairly accurately by determining the size of the egg and the volume liberated. The 

 eggs average about 0.07 mm. in diameter, and the volume liberated at single intervals 

 is from 2 to 4 cc. If the eggs were placed side by side in a line it would require 142 

 to make a centimeter, and 142 such lines, or 20,164, to cover a square centimeter with 

 eggs one layer thick. A cubic centimeter would contain 142 layers, or 2,863,288 eggs, 

 if placed one directly upon another. As a matter of fact, however, this does not occur, 

 for the eggs settle in the depressions formed between those of the layer beneath, which 

 results in adding some 500,000 more to the total number. But owing to the various 

 errors that occur in making the measurements, such as the presence of foreign matter 

 and the buoying property of the water, which prevents the eggs from coming into actual 

 contact with each other, it probably more closely approximates the truth to assume 

 that there are 2,863,288 eggs to the cubic centimeter. This would mean, therefore, 

 that a mussel during a single act of spawning liberates from 5,000,000 to 12,000,000 

 eggs. By comparing the volumes of water displaced by the mantle and mesosoma of 

 large mussels 3K to 3^ inches long, before and after spawning had occurred, the 

 author found the difference to be a little less than 10 cc, which would indicate that 

 large specimens are capable of producing as high as 25,000,000 eggs. 



The spermatozoa are pin-shaped bodies when observed under the low power of 

 the microscope. Under high magnification the head appears as an oval body, with the 

 small end terminating anteriorly in a conical protuberance. At the base of the conical 

 structure are two small, doubly refractive bodies that stain deeply with janus green 

 in the living element. A long slender vibrating tail projects from the opposite end of 

 the head (fig. 181a, p. 188). In size the head measures 5 microns long and has a width 

 of 2.5 microns. The tail has a length of 35 microns. 



The egg, before leaving the follicle, is more or less spherical in form, with a diameter 

 of 0.07 mm. With transmitted light its color is a pale brownish-yellow. A distinct 

 vitelline membrane no less than 1 micron in thickness envelops the egg, while the 

 center is occupied by a large germinative vesicle containing a conspicuous nucleolus 

 (fig. 18 1 b, p. 188). The portion of the egg outside the germinative vesicle is filled with 

 fine yolk granules that render the egg more or less opaque, especially after the germi- 

 native vesicle breaks down, which occurs at the time of spawning (fig. i8id). 



No microphile is visible in the ovarian egg, but if the egg is slightly crushed under 

 a cover glass, it becomes balloon-shaped and an opening appears at the tapering end. 

 Yolk granules flow out from this point of exit, constantly vibrating with the Brownian 

 movement. If pressure continues to be applied to the cover glass the nucleus will also 

 slip out through the opening (fig. 181c). The ripe egg, just before it is expelled from 

 the main oviduct, or immediately after it comes in contact with the sea water, under- 

 goes considerable change internally. While externally it retains a form that varies 

 between a long oval and an imperfect sphere, internally the germinative vesicle breaks 

 down and forms the spindle of the first polar body (fig. 178, p. 184). This explains 

 why no nucleus is visible in the freshly laid egg, as noted by Wilson (1887) and Wil- 

 liamson (1907), rather than the presence of abundant deutoplasmic granules to which 

 they attributed the fact. Unless fertilized within three or four hours after extrusion, 

 the eggs die without passing beyond the stage of the first polar spindle 



