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Part III. — Sixteenth Annual Report 



The zona radiata of the mature pelagic egg is thus seen to be very 

 delicate. It is, however, by itself heavier than sea-water, and sinks in 

 it ; this may be shown by crushing a number of ripe eggs and washing 

 away tlie yolk, and then throwing the capsules into the water. 



That the hyaline enlargement at maturation is dilierent from the 

 ordinary process of growth is obvious, and it takes place with comparative 

 rapidity. The growth of the egg during the formation of yolk takes 

 many months, in which time the egg of the plaice, for instance, increases 

 slowly and gradually in diameter from about 0*2mm. to about 1*1 mm. ; 

 but in the final stage tJiere is a rapid increase from the latter to I'Smm. 

 The number of transition stages to be found in a ripe ovary is extremely 

 small. Almost all the eggs consist either of the large opaque kinds or of 

 the mature translucent kinds, three or four times their size. This can be 

 readily shown by examining the ripe ovary, either in the fresh condition 

 or after boiling or preservation in hardening agents. If a piece of a quite 

 ripe ovary be dropped into, say, picric acid solution, the distinction 

 between the two sizes, the rarity of intermediate stages, is evident. This 

 fact was strongly impressed on my mind in making the investigation 

 on the fecundity of fishes, when over 400,000 eggs were separately 

 enumerated. If the process was slow and comparable to growth, one 

 should see a great number of eggs at intermediate stages both as regards 

 size and appearance. Whether the change takes hours or days is 

 uncertain. 



Transition stages can, however, be found by careful examination of a 

 large number of eggs. Most of them present essentially the same features 

 as the opaque eggs ; but they are somewhat larger, the yolk-spherules are 

 larger, and the whole ovum more translucent, resembling the appearance 

 of the mature demersal egg, such as that of the herring. If oil globules 

 exist, they now begin to appear on the surface of the viteUus. These eggs 

 resemble opaque eggs which have been soaked in a weak solution of 

 common salt, and they indicate incipient dissolution of the fabric of the 

 egg. My sections of pelagic eggs do not include any that can be with 

 certainty assigned to this stage. In sections of the ripe ovary of a bib, 

 there is an egg measuring about 0"53mm., in which the germinal vesicle 

 is somewhat excentric, with the nucleoli very minute, and the yolk partly 

 in the condition of large irregular spherules, but mostly homogeneous and 

 somewhat colloid-looking. This homogeneous yolk resembles what is 

 seen in the mature eggs in the same section, except that it is more deeply 

 stained and solid-looking. Other eggs show somewhat similar changes ; 

 in some cases the germinal vesicle is flask-shaped or pyriform and large ; 

 in other cases normal yolk-spheres are present. But this ovary was not 

 preserved until a considerable time after the death of the fish, and it is 

 not improbable that the appearances mentioned are due to post-mortem 

 change. Other transitional stages will be referred to later. 



From many considerations it is clear that the cause of the change in 

 the pelagic ovum at maturation is the entrance of watery fluid. This is 

 shown by the transparency, the rapid increase in volume, the solution of 

 the yolk-spherules, the diminished specific gravity. The contrast between 

 the opaque and translucent eggs after boiling, or after immersion in fresh 

 water or hardening agents, shows the same thing. The opaque eggs after 

 boiling or immersion in water become intensely chalky white and harder ; 

 the ripe eggs remain soft, and become only slightly milky or opalescent. 

 In picric acid solution the opaque eggs become brown, while the ripe eggs 

 remain straw-coloured ; in osmic acid solution the opaque eggs become 

 black, and the translucent eggs brownish grey. Between the fingers or 

 in manipulation on the slide, the difi'erence in consistency is obvious. 



