THE FULLY GROWN OVUM BEFORE MATURATION. 45 



thread, which is easily overlooked on account of its extreme fineness. The 

 main piece, m, of the tail is about half as thick as the middle piece. It 

 gradually tapers and ends abruptly at the beginning of the still finer and very 

 short end piece, e. 



The spermatozoa, when free in the fluids in which they normally occur, are 

 capable of active locomotion. This is achieved by means of the tail, which acts 

 as the swimming organ by vibratory undulations which drive the spermatozoon 

 along, head foremost. The tail has often been compared to the flagellum which 

 serves as. the locomotive organ for many of the unicellular organisms. 



The Fully Grown Ovum Before Maturation. 



The structure to be here described is not the true sexual element, but is only 

 the modified germ-cell which has accomplished its period of growth and is ready 

 to be transformed into the genuine female sexual element. This transformation 

 is called the maturation, and is accomplished essentially by the expulsion of the 

 so-called polar granules. The full-grown mammalian ovum is found in the 

 ovary in the center of the discus proligerus of the Graafian follicle. It measures 

 from o. 10 to 0.15 mm. in diameter. It is approximately spherical. In some 

 cases observers have found a very delicate vitelline membrane covering the pro- 

 toplasm. Others have failed to observe this. Outside there is a thick envelope 

 measuring from 0.02 to 0.03 mm. in diameter and known as the zona pellucida or 

 radiata. Against the outside of the zona rest the cells of the discus proligerus 

 which constitute the so-called " corona radiata." The nucleus is large, spherical, 

 contains a distinct nucleolus, and always occupies an eccentric position.* The 

 protoplasm of the cell is large in amount, granular in appearance, forms a dis- 

 tinct reticulum, and contains a greater or less number of yolk granules which 

 vary considerably in character, size, and distribution in different mammals. 

 They are usually more or less concentrated in the central portion of the ovum, 

 leaving the outer portion, known as the protoplasmic zone, more or less free. 



The Human Ovum. — The full-grown human ovum is distinguished among 

 mammalian ova for the clear development and ready visibility of all its parts, a 

 peculiarity due chiefly to the small amount of the yolk and the fewness of the 

 fat granules it contains. Fig. 2 represents an ovum from a woman of thirty 

 years. The specimen was obtained by ovariotomy, examined and drawn in the 

 fresh state, being in the liquor folliculi. The specimen gave the following meas- 

 ures: The diameter of the whole ovum, including the zona radiata, 165-170 fi; 

 thickness of zona, 20-34 !'■ ! perivitelline fissure, 1.3 // ; the clear outer zone of the 



* The nucleus was formerly termed "germinal vesicle" ; the nucleolus, " germinal spot." 



