74 



THE BIOLOGY OF AN ANIMAL. 



individual is hoth male and female^ producing both eggs and 

 sjDermatozoa. The ova arise in special organs, the ovaries^ the 

 spermatozoa in spermaries or testes. 



The ripe ovum (Fig. 33, B) is a relatively large spherical 

 cell, agreeing closely with the e^g of the star-lish (Fig. 12), but 

 having a thinner and more delicate membrane. It is still cus- 

 tomary to apply to ova the old terminology, calling the cell- 

 substance ly'dellus^ the membrane vitelline membrane, the nucleus 

 germinal vesicle, and the nucleolus germinal spot. 



The ripe spermatozoon (Fig. 33, 6^) is an extremely minute 

 elongated cell or filament thickening towards one end to form 

 the head (71), which contains the nucleus of the cell enveloped by a 

 thin layer of protoplasm. This is followed by a short ' ' middle 

 piece ' ' (m) to which is attached a long vibratory fiagellum or tail 

 (t). The tail is virtually a long cilium (p. 31), which by vigorous 

 lashing di'ives the whole cell along head-foremost, very much as 

 a tadpole is driven by its tail. 



Since the ovaries and spermaries give rise to the germ-cells, 



they are called the essential organs of 

 reproduction. Besides these, ZtimhricuSj 

 like most animals, has accessory organs of 

 reproduction which act as reservoirs or 

 carriers of the germs, assist in securing 

 cross-fertilization, and minister to the 

 wants of the young worms. 



Essential Reproductive Organs. The 

 ovaries are two in number and lie one on 

 eitlier side in the 13th somite attached to 

 the hinder face of the anterior dissepiment 

 {ov, Fig. 29). They are about 2"^"^ in 

 length, distinctly pear-shaped, and at- 

 tached by the broader end (Fig. 32). The 

 narrow^ extremity contains a single row of 

 ova and is called the eqq-strinq ies). In 



Fig. 32.— The ovary, much , . , . ^^ ^ \ J 



enlarged, /j, the basal part; this the ova are ripe or nearly so; behind 



a body of the ovary con- ^|^ g|^^^g ^^ -^^^^ ^l^^g^ ^^^^ ^^^ 

 taming immature ova; es, ^ "^ 



egg-string; oi\ ripe ovum immature, till these are lost in a mass of 



rea > to a off. nearly undifferentiated cells {primitive 



ova), constituting the great bulk of the ovary. Each of these, 



ov 



ea 





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