302 ELEMENTARY BIOLOGY. [CHAP. 



Immature spermatozoa are at times present in 

 abundance. They can be recognised by the relatively 

 large size of the 'head 1 , which is oval and can be 

 distinctly seen to lodge a small nucleus. (Cf. Lum- 

 bricus and Rana.) 



b. The ripe ovum. Examine under a high power. 



a. Its shape; usually round, more rarely oval. 



/3. The absence of a distinct vitelline membrane. 



y. The protoplasm ; consisting of a central uniformly 



granular vitellus, which graduates peripherally 



into a superficial clear layer. 

 S. The germinal vesicle; a large round structure, 



usually containing one immense germinal spot. 



L. Development. (The Pond Snail, Lymnaus stagnalis.) 

 See p. 278. 

 Examine the developing eggs from time to time, and look 

 especially for — 



a. The fertilized ovum {oosperm) during segmentation. 

 It divides up into two sets of cells — a smaller 

 rapidly dividing clearer set, and a larger slowly 

 dividing yolk-laden set (the presence of the yolk 

 gives these an opaque coarsely granular appear- 

 ance). Look out for polar bodies (see p. 278). 



b. The same at the close of segmentation. Looked at 

 from the outside the larger cells appear as a dark 

 mass, lying within the smaller more transparent 

 ones, which have now completely overgrown them. 



c. The gastrula phase, immediately following upon b. 

 Examine from beneath and note 



a., the blastopore; a small slit-like orifice on the 

 flattened under surface, 



