106 GASTROPODA. 



The actual egg of the Gastropods, if not specially large, is fairly 

 rich in yolk which is often yellow, but occasionally of some other 

 colour (blue-green in Patelln) ; this frequently renders the egg quite 

 opaque. A clear protoplasmic region can frequently be distinguished 

 from a more opaque region laden with yolk, the difference between 

 the animal and the vegetative pole being thus indicated (Fig. 40 A 

 and B). 



In some Gastropod eggs there is less yolk than in others. 1 \ilu- 

 clina may be cited as an extreme case on the one hand, and ya*xn. 

 and FVBUS on the other. The egg itself is usually surrounded by a 

 clear viscid mass which , in its turn, is again enclosed in a transparent 

 envelope. It has already been mentioned that other envelopes may 

 be added, and that several eggs may be enclosed in a common 

 capsule. 



2. Cleavage and Formation of the Germ-Layers. 



In spite of the great number of forjns among the Gastropoda and 

 the different development of the several divisions, we can, in every 

 case which has been investigated, recognise a common plan in the 

 cleavage of the egg, although at times this is more or less obscured 

 by modifications introduced by the variations in the amount of the 

 yolk present. 



In this respect we have resemblance to the Lamellibranchs, but 

 the course of cleavage itself is different in the Gastropoda. The 

 phenomena of cleavage have been studied in a large number of 

 Gastropods and may therefore be considered as pretty accurately 

 understood. As early as 1850, the cleavage of the Gastropod egg- 

 was described by WAKNECK (No. 130), very completely, considering 

 the time at which he wrote. And since then it has been investigated 

 by a number of zoologists, among whom we may mention FOL, 

 BOBRETZKY, EARL, MARK, BLOCHMANN and others (see the literature 

 appended to this chapter). 



In all Gastropods, as far as is known, cleavage is total ; at first it 

 may be equal, but it very soon becomes unequal. The egg, in many 

 cases, is divided up into two large blastomeres of almost equal size 

 by a median groove which cuts it below the polar bodies (Fig. 40 A). 

 A second furrow, which is also meridional, divides the egg into four 

 almost equal blastomeres (B, I- IV). These four cells, owing to the 

 nature of the second cleavage, often lie in such a way that two are 

 in contact with one another in the centre of the egg and thus 



