130 MR J. T. CUNNINGHAM ON THE 



Order VI. PLECTOGNATHI. 

 Fam. 1. Sclerodermi. 

 „ 2. Gymnodontes. 



The development of these has not been studied. 



The Maturation and Fertilisation of the Teleostean Ovum. 



In considering the subject of the phenomena which take place in the ripe 

 Teleostean ovum immediately after its separation from the parent, two ques- 

 tions chiefly excited my curiosity, neither of which have I yet solved to my 

 complete satisfaction. These questions refer to the account of the phenomena 

 which has been given by Professor C. K. Hoffmann." The first is, Is there 

 any foundation for Hoffmann's statement that the first segmentation spindle is 

 directed radially, and divides into a superficial nucleus which belongs to the 

 archiblast, and a deeper one which belongs to the periblast % The second is, 

 Can we trace in the fish ovum the transformations of the nucleus which 

 accompany the expulsion of the polar bodies, and compare these transforma- 

 tions with those which E. van Beneden t has described in Asearis megalocephala. 



The subject of the last question will be considered first. The investigation 

 of the matter is one of considerable difficulty. It is necessary, in the first 

 place, to have a plentiful supply of healthy living specimens of some species 

 with pelagic ova ; and in the second place, to have at command the most 

 approved appliances and reagents for their microscopic examination. The first 

 opportunity I had of making the attempt was in May of the present year, when I 

 had a number of ripe Pleuronectes limanda alive in the aquarium of the Station. 



In the ripe ovum of P. limanda, immediately on its escape from the ovary, 

 the zona radiata is in immediate contact with the ovum. The condition of the 

 ovum is shown in Plate III. fig. 1. There is no doubt that the eggs of all the 

 species of Pleuronectes and Gadus are closely similar except in size, and Ryder 

 is in error when he indicates a peri vitelline space in his figure of the ripe newly 

 shed ovum of the cod. There is a layer of protoplasm round the ovum in the 

 neighbourhood of the micropyle, which thins out at the pole opposite the 

 micropyle. In the living egg, within half an hour after it is shed, whether 

 milt be added to the water in which it is contained or not, the expulsion of a 

 transparent spherical polar body through the micropyle can be readily observed. 

 Its appearance is shown in Plate II. fig. 10, taken from an unfertilised ovum, 

 and Plate III. fig. 4, three hours after fertilisation. At this latter stage the 

 perivitelline space has begun to appear ; it develops first in a ring round the 

 micropyle as a centre. The protoplasm, immediately after the ovum is shed, 

 begins to collect at the micropylar pole of the ovum, and this process begins 



* See Vcrhandelingen der Tconink. Alcad. der Wetenschappen, Th. xxi., Amsterdam, 1881. 

 t La Maturation, Fecondation, etc., et hi Division Celhdaire, Paris et Gaud, 1883. 



