1S86.] OVUM OF LEPIDOSIREN. 2!H 



large share in the building up of the ovum ; in later stages, but 

 before the follicular cells have dwindled down to a single layer, there 

 is a considerable proliferation and migration inwards of these cells 

 (Plate XXVIII. figs. 5, 6) ; finally, when the ovum is surrounded by 

 ouly a single layer of follicular cells, there is still here and there a 

 proliferation of these cells. 



VI. Resume. 



The observations recorded in the present paper may be briefly 

 summed up as follows. 



The ovum is in the earliest stages observed composed of a mass of 

 deeply staining granular protoplasm ; it contains a germinal vesicle 

 limited by a distinct membrane, inside of which is a peripheral layer 

 of germinal spots ; the ovum is separated from the surrounding 

 follicular epithelium by a thin structureless membrane. The follicular 

 epithelium is composed of a single layer of cells, each with a large 

 deeply staining nucleus, which retains the same character through- 

 out the whole development of the ovum. In the second stage the 

 protoplasm of the ovum is arranged in a reticulate fashion ; the 

 germinal vesicle is not defined by a limiting membrane ; within the 

 vitelline membrane, which is still delicate and structureless, is a well- 

 defined zona radiata with closely arranged vertical pores. 



In the third stage the external vitelline membrane is much thicker 

 and shows a radial striation, the pores being further apart than 

 those of the external zona radiata ; the latter is commencing to 

 disappear. The formation of yolk is in active progress and takes 

 place within the ovum, though not necessarily in its peripheral 

 layers. 



In the more mature ova the yolk occupies the whole extent of the 

 ovum ; occasionally a peripheral layer is to be distinguished from a 

 central mass ; the vitelline membrane alone is present, and is 

 excessively thin and apparently homogeneous, with no trace of radial 

 striation. The germinal vesicle has undergone certain changes, 

 chiefly in the centiipetal movement of the germinal spots. 



A single section frequently contained ova of all the above mentioned 

 stages ; besides these there were present in the ovary a vast number 

 of ova which undergo a diflFerent development ; in size these ova 

 were generally larger than those of Stages 2 and 3. The follicular 

 epithelium of the ova is composed of a single layer of large, more or 

 less columnar cells filled with granules exactly hke the yolk-particles ; 

 these cells proliferate and migrate into the interior of the ovum ; 

 eventually they disappear: the yolk of these ova appears to be, at 

 least in a great measure, formed by these cells, whose contents are 

 round granules quite indistinguishable from the yolk. The follicular 

 epithelium rests directly upon the ovum and is separated from it by 

 no membrane. The earlier stages of these ova seem to indicate that 

 they are formed by the fusion of a number of germinal cells. 



At a later stage these ova were indistinguishable from other 

 mature ova ; the follicular epithelium decreases in importance, and 

 a delicate membrane is formed between it and the ovum. 



