of the Fishery Board for Scotland. 



119 



irregularly rounded or lozenge-shaped, flattened, with a scaly margin 

 and a large prominent nucleus, and they measure about 0'009mm. in 

 diameter. The surface of a large immature ovum of this species is 

 equal to about 4 square millimetres, and the follicular cells covering 

 it will therefore number close upon 50,000, and from the large size of 

 the nucleus and the scanty cytoplasm, it is obvious that the follicular 

 layer must form a potent secreting organ. The cells resemble the 

 epithelioid cells from a serous membrane. 



The assumption is also reasonable that the fluid which enters the ovum 

 at maturation is essentially the same as the free watery fluid found in the 

 cavity of the ovary bathing tlie ripe eggs after the latter escape from the 

 follicles ; and which, therefore, probably represents a surplus secretion by 

 the follicular epithelium after the escape of the ovum, and its abundance 

 will ensure that each egg obtains sufficient to saturate it. As to the mode 

 by which the fluid enters the ovum, it seems to me not improbable that 

 the process, at least in the later stage, is essentially osmotic, the watery 

 solution of crystalloid (chloride of sodium) entering through the egg- 

 membrane, and an osmotic equivalent of albuminous matter passing out. 

 The question remains as to whether this watery secretion of the follicular 

 cells is only formed towards the close of ovarian growth, or is the normal 

 secretion. If it is the same as the normal secretion of the follicular cells, 

 and represents the form in which these cells prepare the pabulum from 

 the blood plasma in the stroma for the ordinary growth of the egg — if 

 this function of the follicle undergoes no modification in the later stages — 

 then it is evident that some part of the ovum itself must undergo a 

 change by which the ordinary assimilative metabolism is brought to a 

 close. According to this view the follicular epithelium continues to 

 perform its normal function of preparing a watery saline solution of 

 albumins which cannot be assimilated owing to the decay of some part of 

 the ovum that governs assimilation. 



It may be worth while considering this theory a little. The great mass 

 of the opaque ovum consists of yolk spherules and granules imbedded, or 

 immersed, in a fluid matrix or menstruum, which forms a multitude of 

 channels between the cortex and the germinal vesicle ; and we know that 

 the formation of the yolk granules goes on throughout the whole of the 

 cytoplasm. In most cases the larger, and therefore older, spherules are to 

 be found some distance below the cortex ; in some cases they are placed 

 nearest the cortex, those in the neighbourhood of the germinal vesicle 

 being small (Fig. 11, PI. II.), and therefore probably newly formed. 

 There must be within the vitellus a continuous circulation of the secretion 

 derived from the follicle, and after the constituents which go to form the 

 more solid yolk-spherules are withdrawn, the excess of water must be 

 returned by the follicular epithelium to the stroma. From what is known 

 regarding the function of the nucleus in the synthetic metabolism of the 

 ceU, it is, I think, certain that it is in the germinal vesicle that we must 

 seek the controlling agency of the general metabolism of the ovum, 

 although the formation of the cortical protoplasm may take place in situ, 

 and become transferred inwards. And, moreover, in the teleostean ovum 

 it is, I believe, the nucleoli that must be regarded as the most obvious 

 active agents. We have seen that they are numerous and large, and are 

 closely applied to the membrane of the germinal vesicle during the period 

 of yolk' formation, while towards the close of ovarian growth many, or it 

 may be all (Fig. 5, PI. III.), of them pass inwards towards the centre of 

 the vesicle, and break down. This change in the nucleoli precedes the 

 complete hyaline enlargement at maturation, and it is not improbable 

 that, when the ovum has attained the specific size and condition, the 



