THE DEVELOPMENT OF A MOLLUSK 33 



capsules, to the number of 50 or 60, each containing about 250 

 eggs, are united into a grape-like cluster generally found under 

 the shell of the Crepidula attached to the stone or other object 

 upon which it lives its sedentary life. The total number of eggs 

 laid at one time by an animal is about 13,000. 



The unfertilized egg (Fig. a) is a nearly spherical single cell 

 consisting of a very small amount of protoplasm surrounded by 

 a relatively larger amount of yolk material, mostly in the 

 form of small globules. Within the protoplasm, in a nearly 

 central position, is found the nucleus of the cell. The whole egg 

 is enveloped by a cell membrane. 



NUCLEUS 



■PIfOTOPUSM 



Fig. a 



The first change which takes place in the egg, preparatory to 

 development, is a migration of the nucleus and protoplasm from 

 a central position toward the upper surface of the egg, the yolk, 

 or deutoplasm, taking its position below it. The egg thus be- 

 comes distinctly symmetrical about a vertical axis (Fig. h). The 

 upper pole, at which the protoplasm is found, is known as the 

 animal pole ; the opposite, or lower, as the vegetative pole, since 



ANIMAL POLE 

 NUCLEUS ->-^<^J^--. 



-PIfOTOPLASM 



