Reproductio 



237 



Fig. 201. Human ovum surrounded 

 by follicular cells. Actual diameter of 

 ovum about 0.25 mm. (C) Cytoplasm 

 containing some yolk; (CR) corona 

 radiata; (F) follicular cells; (N) nu- 

 cleus; (ZP) zona pellucida. (After 

 Nagel. Courtesy, Neal and Rand: 

 "Chordate Anatomy," Philadelphia, 

 The Rlakiston Company.) 



sperm chromatin becomes joined with the chromatin of the egg nucleus. 

 The "maturation" process through which all germ-cells pass reduces 

 their chromatin to approximately half that contained in body cells, 

 so that the union of sperm chro- 

 matin and egg chromatin provides 

 the fertilized egg with a nucleus 

 containing the full complement of 

 chromatic bodies (chromosomes) 

 characteristic of all body cells of 

 the animal. Therefore the fertilized 

 egg, although the product of two 

 cells, possesses the mechanism of a 

 single cell. It possesses no visible 

 structures which would adequately 

 account for its development into a 

 large complex animal like the par- 

 ent animals. Compared to such 

 cells as those of muscle and ner- 

 vous tissue, it is strikingly devoid 

 of visible special mechanism. Yolk 

 is characteristic of eggs, but yolk 

 is an inert food substance, not a mechanism. 



Developmental Potentialities. The motile and aggressive sper- 

 matozoon might seem to be the essentially "animal" body in develop- 

 ment, while the relatively large unfertilized egg, burdened with inert 

 yolk, would appear rather as a passive and vegetative thing. But in 

 normal development the spermatozoon merely imparts the stimulus 

 which initiates development and provides for inheritance from a male 

 parent. Experimentation has proved that artificially applied mechan- 

 ical or chemical agencies may stimulate an egg to develop and produce 

 a characteristic adult without the assistance of a spermatozoon. 

 Obviously, however, such an adult inherits only from a mother, since 

 its "father" may have been a mere needle-prick or a change in osmotic 

 pressure of the external medium. 



Exit of Sperm and Eggs. The sperm is usually carried by ducts 

 which lead from the testis to the exterior, but in cyclostomes and some 

 bony fishes it is discharged from the testes into the body-cavity 

 and finds exit through abdominal or genital pores which pierce the 

 body-wall. 



Ova are usually liberated from the surface of a solid ovary (Fig. 196) 

 into the body-cavity, whence they pass into oviducts which lead to the 

 exterior. In cyclostomes and some bony fishes, the eggs pass out 

 through abdominal pores. In other bony fishes, the ovary is hollow 



