158 MOLLUSKS. 



I. Segmentation. Place the watch glass upon the stage 

 of the microscope, and examine the eggs with IOAV power. 

 If a freshly laid capsule has been obtained, the oosperm 

 of each egg \vill appear perfectly spherical in form, and 

 slightly yellowish in color, from the large number of 

 minute food-yolk granules distributed through it. If it 

 be kept covered with water, and watched at intervals of 

 half an hour or less during the first day, and at intervals 

 of half a day thereafter until hatched, the following 

 changes may be confidently expected : 



The oosperm is practically a single large cell, formed 

 by the fusion of two specialized reproductive cells called 

 ovum and sperm. The fusion of the two we have 

 already called fertilization, and have said that it is essen- 

 tially the same process in all animals except the pro- 

 tozoans. Fertilization necessarily precedes and initiates 

 the changes which we are now to study. 



The oospeima divides, and becomes two spherical cells 

 instead of one ; after a short quiescent period, the two 

 divide, and become four ; after another short period, the 

 four divide, and become eight ; and so on, with decreasing 

 regularity. This process is called segmentation. 



If all the eggs in a freshly laid capsule be examined, 

 some of the oosperms will be sure to show a lighter and 

 a darker side. This is owing to a greater accumulation 

 of yolk granules at one side of the sphere. Some will 

 show, also, one or two minute transparent polar bodies 

 pushed out from the middle of the lighter side. That pole 

 of the sphere at which they appear is called the formative 

 pole. In segmentation, a groove first appears at the forma- 

 tive pole, extends meridionally around the oosperm, and 

 deepens until two separate cells are formed. These cells 

 appear to partly coalesce again, before the next division. 

 Then a second groove starts at the formative pole, and ex- 

 tends meridionally around the two cells at right angles to 



