ox THE EOEIIATIOX OF THE EGO IN THE AXXHLOSA. 
621 
from the first in all instances, and this macula cannot, therefore, be regarded as the 
centre of its formation.” Other naturalists, however, have given a very different account 
of the process of egg-development ; and even Dr. Thomson himself, in that part of the 
same article which refers to the Acalephse, does not figure or describe the Purkin- 
jean vesicle as appearing until the second stage. Gegenbaue also gives a very similar 
account of what takes place in Thaumantias. He says, “ One sees, moreover, often even 
in one and the same animal, that some of the cells filling the ovary increase in size, the 
membrane raises itself more considerably from the nucleus, and at this time molecules, 
generally arranging themselves round the nucleus, begin to differentiate themselves in 
the originally homogeneous cell-contents. Only two or three cells of the primitive 
ovarian parench^me pass through these changes, and thus become egg-germs; their 
growth proceeds further and fui'ther, and the contents of the egg-cell now consist of a 
finely granular substance, in the centre of which a transparent nucleus (the germinal 
vesicle) lies imbedded.” Among the Mollusca, according to a very accurate observer, 
M. Lacaze Duthiees, the egg of Lentalium arises from a modified epithelial cell, the 
nucleus of which becomes the Purkinjean vesicle. These instances, however, are foreign 
to our immediate subject; but even among the Annulosa similar observations are upon 
record. In describing the ovaiy of Argulus, Leydig* says, “The smallest eggs are clear 
round cells, whose vesicular nucleus contains many nucleoli. They alter themselves 
gradually into eggs, and pass slowly from a circular to an oval shape, &c.” Again, in 
lAmiihis, GEGENBAUEf expressly describes and figures the egg as arising from the modi- 
fication of a single epithelial cell, the nucleus of which becomes the germinal vesicle J ; 
and in Cijjpris the egg has been described as having a similar origin. In his paper on 
Argos persiciis^, Dr. C. Hellee says, “ In its original form the egg appears as a colour- 
less cell, with a transparent vesicular nucleus and finely granular contents. In more 
advanced eggs the finely granular mass is in greater quantity, and of a yellowish colour ; 
an evident germinal vesicle is now present, in which the round germinal spot is clearly 
nsible.” Finally, if Meissnee is coinect, the membrane of the original cell becomes the 
■vitelline membrane also in Mermis and Gordius, and in some of the Nematoid worms. 
If these observations, or any of them, are coiTect, it is evident that we have in the 
Annulosa, as even in the animal kingdom generally, two essentially distinct types of 
egg-development, since the original epithelial, or at least ovarian cell, which becomes 
the whole egg in Argulus, Limulus, Argos, Mermis, See., forms in other cases only the 
Purkinjean vesicle ; so that, taking the undifferentiated ovarian cells as our starting- 
point and standard of reference, the Purkinjean vesicle in certain animals corresponds 
to the whole egg in others. In other words, we can no longer regard the “ eggs ” of all 
Annulose animals as being homologous with one another, but we must consider that, as 
* SiEBOLD and Kollikeb’s Zeits. f. Wiss. Zool. 1850, p. iv. 
t Anatomisclie Enters, eines Limulus, mit besonderer Beriicksichtigung der Gewebe. Halle, 1858. 
X Loc. cit. pi. 1. fig. S. 
§ Aus dem xxx. Bande des Jabr. 1858 der Sitz. der Math.-naturw. Cl. der Kais. Ak. Wien. 
4 p 2 
