28 THE SCALLOP FISHERY 



takes place externally to the parent, the scallop develops by the normal 

 process of unequal cell division, and its subsequent growth as far as 

 the prodissoconch stage is similar in nearly every respect to the devel- 

 opment of the clam, oyster and quahaug. 



The Polar Cells (Figs. 4, 5). --The first noticeable change in the 

 external appearance of the egg occurs about thirty-three minutes after 

 it is laid. At one part of the egg, which from this time forth becomes 

 the so-called animal pole or region of the greatest activity, appears a 

 small translucent globule, Mo the diameter of the egg. This is known 

 as the first polar cell, and is soon followed by a second body of similar 

 nature, which pushes out behind the first in such a manner as to 

 separate it from the egg. Both adhere to the egg by protoplasmic 

 strands, such as described by Drew (1). With Venus mercenaria (the 

 quahaug or hard-shell clam) the polar cells form beneath a thin mem- 

 brane, and are held to the egg by strands from this source. Such a 

 covering is not well marked in the scallop egg, which appears naked, 

 and the protoplasmic strands may possibly have a different origin. 

 The polar cells contain no yolk granules, as is shown by their trans- 

 parent appearance. They remain with the egg through all the varied 

 stages of cell division, and can be seen still adhering to the first ciliated 

 larvae, evidently disappearing during the early swimming stage. 



The Tolk Lobe. About ten minutes after the first polar cell is 

 formed, the opposite side of the egg, now known as the vegetative pole, 

 elongates, giving to the egg a pear-shaped appearance. The constric- 

 tion at the small end is the so-called yolk lobe (Fig. 5) which forms a 

 few minutes previous to cleavage. Pecten irradians differs somewhat 

 from Pecten tenuicostatus in regard to the time of formation of the 

 yolk lobe. In the case of the latter species, Drew (1) has shown that 

 the yolk lobe appears previous to the polar bodies, and that it becomes 

 prominent when the second polar body is formed, only to disap- 

 pear and again to become prominent when the egg cleaves into 

 two cells. The yolk lobe in the former was not seen until after the 

 formation of the polar cells, and not until just before the first cleavage 

 did it become markedly prominent. It forms in about three minutes, 

 and is completed one minute before the first cleavage takes place. 



The First Cleavage. Soon after the formation of the yolk lobe and 

 the differentiation of the egg into the animal and vegetative poles a 

 constriction takes place parallel to the longitudinal axis of the egg, 

 dividing the broad end into two unequal cells, the smaller one-half the 

 size of the larger, with the polar bodies between them (Fig. 7). The 

 actual time consumed from the beginning to the completion of the first 

 cleavage varies from two to twelve minutes, but usually it takes about 

 three minutes to effect the change. This first division occurs forty-six 

 minutes after the egg is fertilized. 



The action of the yolk lobe during this division is somewhat peculiar. 



