14 DISCOPHOR^. ' Part III. 



so as to appear about three times the diameter represented here, the yolk-cells 

 would have the size and ajjpearance of those in Fir/. 24 y. The yolk-sac is so 

 exceedingly thin that the yolk appears to extend to the very periphery of the 

 egg. At this stage of growth the yolk has no longer the trans25arent, colorless 

 appearance of the earlier periods, but presents a Ijluish-gray color. From this time 

 forward there is but one remarkable change noticeable in the egg, and that is the 

 dissolution of the yolk-cells and their re-develo^iment. That this does occur is proved 

 by the fact, that in a fully grown egg {Fig. 22) the yolk-cells {//) are smaller than 

 those of the last phase mentioned {Fig. 21) ; and to demonstrate that they are not 

 the mesoblasts of the cells of the previous period, it is enough to say that these 

 cells were not mesoblasted. 



For a short time after this, the egg would seem to increase in size, but not 

 as an entire egg. The Purkinjean vesicle (PI. X^ Fig. 23 j)) bursts, and yet the 

 space occupied by it remains clear, and the Wagnerian vesicle {Fig. 23 tv) continues 

 intact, and might be mistaken for the Purkinjean vesicle, were it not for its pecuhar 

 ajipearance, by which it may be recognized when compared with other Wagnerian 

 vesicles of undoubted character and relations. The yolk-cells, at this period, are 

 larger than ever, and have an ajiparent diameter, under this magnifying power, of 

 about one thirtieth of an inch, or in reality g o'o o of an inch in diameter. The 

 vitelline sac is very thick, a peculiarity also noticeable in the ripe egg of another 

 genus, Cyanea (see PI. X. Fig. 2 v), but never in the eggs of any of the naked- 

 eyed Medusas. Finally, the Wagnerian vesicle bursts, and leaves a homogeneous 

 clear space (PI. X". Fig. 24: p) in the centre of the egg. To Avhat degree this clear 

 space is filled up, or whether it disappears altogether during segmentation, we are 

 not able to state; for we have not seen the segmentation of the yolk either in 

 Aurelia or in Cyanea.^ 



The Planula of Aurelia flavidula.^ After segmentation there is some variation 

 in the age at which the young leave the ovary and enter the pouches of the oral 

 appendages; for they do it by their own strength, being provided with vibratile 

 cilia all over the body {Mg. 25). Some go out before they have lost their globular 

 shape {Fig. 25), and others remain until they have become oval {Fig. 30), or even 

 quite elongate {Figs. 31 and 32) ; but at no time do they leave in an unsegmented 

 state.^ There would seem to be considerable variation in size among the young, 



^ If we may judge from Siebold's figures (Neueste ^ See Vol. III. p. 80 for the meaning of the 



Schriften der Naturforschenden Gesellschaft in Dan- word planula as used here. 



zig, 1839, Tab. 1, Figs. 3, 4, 5% and 5'b) of the ^ Siebold (Neueste Schrift. etc., Danzig, 1839, 



segmentation of the yolk of Medusa (Aurelia) aurita. Fig. 18) says in regard to Aurelia aurita, that the 



we should say that this clear space became obli- eggs escape from the ovary and reach the pouches 



terated during the process. without the help of vibratile cilia; but then again, 



