144 J. T. CUNNINGHAM. 



The follow iug stages are to be seeu iu sections made I'rom 

 the ovary of a plaice killed on August 17th. The development 

 of yolk was advanced in many of the ova. The best prepara- 

 tions are from portions fixed with chromosmic mixture or 

 Kleinenberg's picro-sulphuric acid. In ova a little larger than 

 those last mentioned — namelv, from about '16 mm. in diameter 

 upwards — in the interior of the germinal vesicle there can be 

 distinguished a central and a peripheral region. In the central 

 region can be seen, even with a low power, indications of 

 stained fibrils, which are wanting in the peripheral region. 

 Examination with higher powers shows that these fibrils form 

 apparently separate lengths or loops, whose direction is very 

 irregular; sometimes they can be seen to be in pairs, and with 

 the immersion lens they have a feathery appearance. These struc- 

 tures clearly, therefore, correspond to those described by Rlickert 

 iu the Selachian ovum, and identified by him as chromosomes. 

 The impression I have obtained by comparing the ova in this 

 condition with the younger is that the vesicle has expanded, while 

 the fibrillar network has remained of the same size, so that a 

 space has been left between the border of the network and the 

 membrane of the vesicle. In the younger ova the nucleoli are 

 all in contact with the membrane of the vesicle, but in later 

 stages they travel towards the centre, and are found in the 

 region of the chromosomes. The substance between the fibrils 

 and in the peripheral region of the vesicle appears to be finely 

 reticular. Fig. 24 shows the germinal vesicle of an ovum "27 

 mm. in diameter from a chromosmic preparation, as seen with 

 Zeiss immersion 2'0 mm., compensation ocular 8. The re- 

 ticular appearance of the achromatic substance is not very 

 evident, the ovum represented being one of those in the internal 

 part of a piece of material, in which the achromatic substance 

 is very pale. In the ovum from which this figure is taken yolk 

 formation had not yet commenced, and only three small 

 nucleoli are seen in the region occupied by the chromosomes. 



When the period of yolk formation sets in, the following 

 changes occur in the germinal vesicle. The membrane becomes 

 thinner and less distinct, and loses its regular contour. In 



