34 THE EARLY DEVELOPMENT OF MAMMALS. 



mammals. The head probably contains a minute body representing a centrosome, 

 although it has not yet been satisfactorily demonstrated in man. 



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 usually 

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

 observers have found a very delicate vitelline membrane covering the protoplasm. 

 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 consti- 

 tute 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 distinct reticulum, 

 and contains a larger or smaller 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. Figure 3 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 measures: The 

 diameter of the whole ovum, including the zona radiata, 165-170;*; thickness of 

 zona, 20-34;*; perivitelline fissure, 1.3/1; the clear outer zone of the yolk, 4-6;*; 

 the protoplasmic zone, 10-21;*; the zone of yolk granules, 82-87;*; nucleus, 25-27;*. 



The corona radiata, cor.r, consists of elongated radiating cells with rounded 

 ends and oval nuclei. The zona pellucida shows a distinct radial striation. This 

 is probably due to the presence of minute canals running through the zona. The 

 ovum proper is separated from the zona by a narrow fissure, the perivitelline space, 



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



