30 



CHORDATE ANATOMY 



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 parent animals. Compared to such cells as those 

 of muscle and nervous tissue, it is strikingly devoid of visible special 



mechanism. Yolk is characteristic of 

 eggs, but yolk is an inert food sub- 

 stance, not a mechanism. 



The motile and aggressive sperma- 

 tozoon might seem to be the essen- 

 tially "animal" body in development 

 while the relatively large unfertilized 

 egg, burdened with inert yolk, would 

 appear rather as a passive and vegeta- 

 tive thing. But in normal develop- 

 , , ment the spermatozoon merely imparts 



Fig. 29. — Human ovum surrounded ^ ... 



by follicular cells. Actual diameter of the Stimulus which mitiates develop- 

 ovum about 0.25 mm. C. cytoplasm ^^^^ ^^^^ provides for inheritance 



containing some volk; CR, corona 



radiata; F, follicular cells; N, nucleus; from a male parent. Expcrmienta- 

 ZP, zona pellucida. (After Nagel.) ^^j^ ^isi^ proved that the egg is fully 



capable of producing a characteristic adult without the assistance of a 

 spermatozoon. Obviously, however, such an adult inherits only from a 

 mother. 



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. 274) 

 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, eggs are 

 liberated into its lumen and pass out by way of a duct which is an extension 

 of the wall of the ovary. 



The genital ducts are usually closely associated with the duct system 

 of the kidneys. Exceptional conditions occur in bony fishes. 



Oviparity, Viviparity, Impregnation. The means whereb>- ovum 

 and spermatozoon are brought together depends on whether the animal 

 is oviparous or viviparous; also on whether the outer envelopes of the 

 egg can be penetrated by a spermatozoon. 



In most oviparous fishes the eggs are impregnated after the genital 

 products have been discharged into the water ("external fertilization"). 

 But oviparous sharks and skates produce eggs whose shells are impene- 

 trable by sperm. Therefore copulation must occur and the egg must be 



