OF THE GERMINAL LAYERS IN GASTROPODA. 5 



be said to have an ovoid shape. The same state of affairs has 

 been observed in Limax by JMark^^''. Thus, before the cleavage 

 process commences, it already shows a tendency to present 

 inequalities in its two lialves. After the elongation, a depression 

 begins to appear in the animal pole near the polar body. It 

 gradually extends downwards until it almost reaches the center 

 of the egg. At about the same time a similar groove with a 

 much sharper angle appears in tlie vegetative pole which proceeds 

 upwards towai'ds the center of the egg. (Fig. o). The depression 

 from the animal pole, however, proceeds much the more rapidly 

 of the two until it reaches about the center of the egg. There 

 it seems to stop, and to meet the approaching groove from the 

 vegetative pole. Thus these two grooves encircle the egg leaving 

 a bridge which for the time being connects the two halves. In 

 the section, this bridge is proved to be the body of the spindle. 

 This first cleavage plane is often described as moving directly 

 from the animal pole to the vegetative in other species of Mollusca, 

 for example, in Umbrella (Heymons)^"^', Unio (Lillie)^''^\ Suecinea 

 (Schmidt) *''^', Limax (Meisenheimer)^^^\ etc. In either case the 

 cleavage results in the total division of the egg into two 

 halves or blastomeres. The plane itself in Siphonaria stands 

 almost transverse to the longitudinal axis of the future adult. 

 The blastomeres thus formed seem at first sight similar in size ; 

 but a careful observation shows that in reality that carrying the 

 polar hodj is a little larger than its complement. This difierence 

 in the size of the two halves is the consequence of the original 

 inequality existing in the natural configuration of the egg before 

 the cleavage. When the final constriction of the blastomeres 

 takes place each of them has a spherical outline, not departing 

 from it even at the point where they come in contact with each 



