302 KLEMENTARY BIOLOGY. [CIIAP. 



Immature spermatozoa are at times present in 

 abundance. They can be recognised by the relatively 

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

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

 briais and Rana.) 



h. The ri_pe ovum. Examine under a higli i)0\ver. 

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

 p. The absence of a distinct vitelline membrane, 

 y. 'Y\\Q protoplasm ; consisting of a central uniformly 

 granular vitelliis, which graduates peripherally 

 into a superficial clear layer. 



S. The germijial vesicle ; a large round structure, 

 usually containing one immense gerjninal spot. 



L. Development. (The Pond Snail, Ly/n/itciis stagnalis.) 

 See p. 278. 

 Examine the developing eggs from time to time, and look 

 especially for — 



a. T\\Q fertilized ovum 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 appearance). Look out 

 iox polar bodies (see p. 278). 



A 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. 



