126 Darwin, and after Darwin. 



and result in the formation of many different kinds of 

 cells, in the order required for constructing the multi- 

 cellular organism to which the whole series of processes 

 eventually give rise. We have now to consider these 

 processes seriatim. 



First of all the nucleus discharges its polar bodies, 

 as previously mentioned, and in the manner here 

 depicted on the previous page. (Fig. 33.) It will be 

 observed that the nucleus of the ovum, or the germinal 



2. 3. 



FIG. 34. Fertilization of the ovum of an echinoderm. (From Quain's 

 Anatomy, after Selenka.) S, spermatozoon ; m.pr., male pronucleus; 

 f.pr., female pronucleus. I to 4 correspond to D to G in the next 

 figure. 



vesicle as it is called, gets rid first of one and after- 

 wards of the other polar body by an " indirect," or 

 karyokinetic, process of division. ( Fig. 33.) Extrusion 

 of these bodies from the ovum (or it may be only from 

 the nucleus) having been accomplished, what remains 

 of the nucleus retires from the circumference of the ovum, 

 and is called the female pronucleus. (Fig. 33. fpn.} 

 The ovum is now ready for fertilization. A similar 

 emission of nuclear substance is said by some good 



