194 BALBIANI^ ON SEXUAL 



brane. Just before sexual union, however, (rarely later), the 

 yolk mass divides in its turn, and, arranging itself round each 

 of the vesicles, gives rise to eggs like those of other Infusoria. 

 "The number of these vesicles, and, consequently, that of 

 the eggs which, at a later period, come to maturity, is not 

 always in proportion to the length of the cylindrical yolk 

 mass. It is thus, for example, that, in spite of their long 

 nucleus, variously bent or twisted, certain Infusoria produce 

 but a small number of eggs. Such is Trachelius ovum, which 

 contains but two at the time of reproduction ; Bursaria 

 truncatella, which has only four at the same period, &c. In 

 other cases, on the contrary, instead of breaking up into 

 only two or four parts, as in the species just cited, the 

 yolk mass divides into fifteen or twenty fragments, each of 

 which forms itself, separating into a complete egg [Prorodon 

 niveus) ." 



" In some Infusoria the vitelline band does not divide 

 throughout its entire extent at each reproductive epoch, but 

 only at one of its extremities, and merely becomes shorter^ 

 without undergoing any further modifications. Such is the 

 case with Euplotes, in which the eggs withdraw themselves 

 at this period from the common mass, ripen separately, and 

 become fit for fecundation.^' 



What has been said may serve to show the more remark- 

 able variations as to size, form, and number of parts under 

 which the ovai'y occurs, and how its three principal modifi- 

 cations merge into one another by numerous intermediate 

 conditions. 



As its contents ripen, the granular mass of the ovary be- 

 comes more cohesive throughout, and at length adheres so 

 closely to the outer membrane, that, when the eggs are ready 

 for fecundation, this appears to be wanting. Such, however, 

 is not really the case in some Infusoria at least, for, in 

 Stylonychia and Stentor, distinct traces of its presence are 

 still discoverable. 



"Another characteristic which the yolk of the ripe egg 

 presents is that of acquiring, under the action of acetic 

 acid, a far brighter tint than at preceding periods ; this 

 color is light-yellow or blue, and never the dirty yellow shade, 

 more or less deep, which this reagent imparts to the vitellus 

 before the ripening of the egg.^' 



The fully developed eggs appear as clear spherical spots 

 amid the darker parenchyma, and are of nearly equal size in 

 the same individual. They are still, however, so transparent, 

 that the addition of reagents is necessary to exhibit their 

 structure. Acetic acid communicates a bluish-grey, or clear 



