85 



opaque white by reflected light and darlv by transmitted Uglit. They represent an 

 excess of fatty matter belonging to the yolk. 



I have not seen the ripe ovum of the sole immediately after its escape from the ovary. 

 But the ova of other flat-fishes have been studied by myself and others immediately 

 after extrusion, and it has been found, e.g., in the cod and the flounder, that at first 

 the germinal protoplasm is not aggregated into a protuberant mass, but is more 

 extended over the surface of the yolk, and that the vitelline membrane is everywhere 

 in direct contact with the ovum. The aggregation of the protoplasm into a distinct 

 germinal mass and the consequent formation of the perivitelline space take place after 

 the extrusion of the ovum, and take place in the same way whether the ovum is 

 fertilised or not. 



I have not particularly studied the spermatozoon of the sole, but have observed it 

 sufiicieiitly to state that it does not difler in structure in any important respect from 

 that of other flat-fishes. The milt is very scanty in quantity and thin and transparent 

 in appearance, but its peculiarities will be considered in connection with the subject of 

 the artificial propagation of the species. I have given a figure of the spermatozoon of 

 the dab, Fleuronectes limanda, PI. XIII, 7, having accidently omitted to make a drawing 

 of that of the sole. In all bony fishes (Teleostei) the head of the spermatozoon is pear- 

 shaped, the pointed end being directed forwards in motion. The long slender vibratile 

 filament or "tail" is attached to the broader end of the head. The spermatozoa are 

 exceedingly small, quite indistinguishable to the unaided eye, but when the fresh 

 milt is placed under the microscope multitudes of them are seen actively lashing 

 themselves about in all directions. The total length of a spermatozoon of the sole is 

 about -j-jfooths of an inch (-07 mm.). 



Fertilisation of the ovum usually takes place immediately after extrusion, and 

 consists in the entrance of a single spermatozoon into the germinal protoplasm. I 

 have not studied the details of the process in the sole's ovum, but have done so in the 

 ova of the dab {Pleuronectes limanda) and another species {PL cynoglossus). The 

 process in the sole is doubtless the same as in these species, and is as follows : Over 

 the centre of the germinal protoplasm there is a minute aperture in the vitelline 

 membrane which is called the micropjie. Through this aperture a spermatozoon passes, 

 and penetrates into the germinal protoplasm. In the latter is the nucleus which, as 

 the ovum matures, becomes indistinct, and can only be made visible by coagulating 

 the protoplasm with acetic acid or some other chemical reagent. This nucleus can 

 also be stained, as it absorbs colouring fluids, such as solutions of carmine, to a greater 

 extent than the surrounding protoplasm. The nucleus before fertilisation is complete 

 passes to the surface of the germ and gives off in succession two small portions of itself, 

 the polar bodies, which are expelled from the ovum. The nucleus after this forms the 

 lemale pronucleus, and the head of the spermatozoon which has entered the germ 

 forms a similar minute body, the male proiuicleus : these two unite into a single 

 nucleus, the segmentation nucleus 



