REPRODUCTIVE CELLS 15 



that millions of them must go astray in the mud, stick fast to objects, be 

 swallowed by oysters themselves as well as by other animals, or become 

 drifted away by the ebb and flow of the tide. 



Eggs are subject, in a less degree, to similar adversities. They have 

 no power of locomotion, and while they may be swept about and kept in 

 suspension for a time, yet, as can be seen by mixing a quantity in a tumbler 

 of sea-water and letting it stand, most of them must sink to the bottom, 

 where many, in the natural course of events, become smothered in moving 

 sediment, crushed, eaten, or otherwise destroyed. To those that remain 

 suspended for some time there are the chances of being carried out to sea 

 on the one side or of being thrown up on the beach on the other. These 

 are types of the first accidents in a long series, that menace the existence 

 of the progeny at every step from the time of liberation from the parent 

 to the time when the more fortunate are themselves occupying the posi- 

 tion of parents. 



The study of fertilization may be advantageously begun by selecting 

 ripe male and female oysters, and, by a process of stripping into tumblers 

 of sea-water, securing first a stock of normal, healthy, unadulterated eggs 

 and sperms. Rougher methods, such as the extraction of the reproductive, 

 organs, or the chopping up of the bodies of the oysters, carry over a mass 

 of undesirable, injured tissues, which interferes by smothering or decay 

 and is difficult to get rid of. Eggs and sperms may be mixed in any pro- 

 portion on a slide, in a watch-glass or in a tumbler, transfer being effected 

 by a medicine-dropper, a glass tube, or, when in large quantities, by pour- 

 ing. The chief care required is in the temperature, for small, isolated 

 portions of water are liable to become quickly warmed above that of the 

 sea. When large quantities of eggs are used and required to be kept for 

 developing stages, the mass of eggs and sperms should be gently stirred 

 for a few minutes, to allow free access of the sperms to the eggs. Then 

 by repeated settling of the eggs, pouring off of impure water and addition 

 of fresh sea-water, the fertilized eggs and segmenting stages may be kept 

 in healthy condition. 



Within a,bout ten minutes after the addition of sperms to a quantity 

 of eggs almost every egg will be found to be dotted with sperms, that are 

 either helplessly clinging to its surface or are energetically attempting to 

 bore their way into it. This they are unable to do, with the exception 

 of the one perhaps, which is fortunate enough to strike the receptive spot 

 (Plate V, fig. 5) or point where the egg membrane is so thin as to permit 

 easy ingress to the protoplasm. The receptive spot in this case is equi- 

 valent to the micropyle in many thick-walled eggs of other animals, and is 

 situated, in pear-shaped eggs, at the point. Fertilization is not completed 

 with this impregnation and disappearance of the sperm into the egg. 

 The protoplasm of the egg furnishes a suitable medium for the carrying 

 out of complex processes that result in the intimate mingling of the chromo- 



