2 74 THE GREAT BARRIER REEF. 



appear as minute granules onl}-, under the same magnification ; and a considerably higher amplifica- 

 tion is requisite to reveal their individual structure. This is then shown to consist of a minute 

 bulbous head and an exceedingly slender flexible hair-like tail, the proportions between the two 

 being much the same as that of the head and shank of an ordinary pin. After a little experience, 

 it will be found easy to distinguish the comparatively coarse granular ova from the cloudy masses 

 of spermatozoa, when placed on the glass slip, with the aid only of an ordinary pocket lens, or even 

 with the unassisted vision. The assistance of the microscope is, however, desirable to insure the 

 most favourable results, and is altogether indispensable for tracing the further development of the 

 embryos. In many instances it will be found that the ova or spermatozoa are not sufficiently 

 matured, or, in the case of oysters purchased in the market, that they have become so deterior- 

 ated by isolation from their native element for too long a period, as to be incapable of eff"ecting 

 reproduction. All conditions being satisfactory, the ova under the microscope should present a 

 clean and evenly-rounded outline, while the vitality of the spermatozoa should be manifested 

 by their active oscillating and vibratory movements. Should the sperm cells fail to exhibit 

 this vitality, their admixture with the ova will prove of no avail. 



In practice it will be found that the number of oysters containing the female elements, or ova, 

 is greatly in excess of those producing the milt, or sperm cells, the average proportion of the 

 sexes in many hundred examples recently examined being one male to six or seven iemales. 

 The small quantitj^ of milt that is required to fertilise a very large number of ova satisfactorily 

 illustrates Nature's economy in this direction. No peculiarities of external structure exist, so far 

 as I have been able to ascertain, that serve for distinguishing between the male and female 

 oyster before being opened. Healthily matured milt and ova having been successfully obtained, 

 portions of each, the ova predominating, may be mixed in a watch-glass half full of sea-water, 

 and well stirred together. The ova, being heavier, will soon sink to the bottom, leaving the 

 spermatozoa diffused, as a cloud, through the water. After an interval of ten minutes the top 

 water may be poured off" or withdrawn with a pipette and fresh supplied, and any fragments of 

 lacerated tissue or tufts of immature milt must be removed with a needle ; these, if left, will 

 decay, and pollute the water. The pouring off" process should be repeated until the top water 

 is quite clear and the bottom consists entirely of fertilised ova. If a small drop of water 

 containing the mingled milt and ova is examined under the microscope at short intervals, 

 some remarkable changes in the form and structure of the ova will soon be observed. 



Almost immediately following upon the admixture of the two elements it will be found that the 

 sperm cells are adhering in numbers by their dilated heads to the delicate capsular investment, or 

 vitelline membrane, of the ova, and that many of them, through the vigorous vibrations of their 

 tail-like prolongations, will have a distinct oscillatory motion. It may also be observed that, through 

 the aperture of the narrower end of the capsule, known as the micropyle, several of the sperm 

 cells have eff"ected an entrance, and have been brought into direct contact with the body of the 



