12 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the membrane cannot therefore be absolutely inelastic, but might be of 
such strength as to act as an inelastic membrane with regard to such 
slight forces as are exerted within the egg during its early development. 
The existence of a membrane of this peculiar character is, however, 
very improbable, and it loses all its explaining power when the egg of 
another rotifer, Melicerta ringens, is taken into consideration. In this 
species the egg is not a regular ellipsoid or oval, but is of an irregular 
shape, one side being curved in profile, the other straight. (See the 
figures of Zelinka, 91.) This form is retained during development 
exactly as in Asplanchna, yet is not explainable on the assumption of 
a membrane. This question is discussed more fully later. 
The cytoplasm of the egg is closely filled with fine yolk granules, 
These are distributed uniformly throughout the egg (except that they 
are not present in the asters), so that there is no visible differentiation 
into regions containing greater and less amounts of yolk material. 
The development of Asplanchna priodonta Gosse was also examined 
for comparison with that of Asplanchna Herrickii. The egg of this 
species is similar throughout to that of Asplanchna Herrickii, save that 
it is smaller. The average dimensions are about 70% by 60 jp. The 
egg of Asplanchna priodonta is shown in Figure 29, Plate 4, drawn to 
the same scale as the figures from Asplanchna Herrickii. 
2. MATURATION. 
The formation of the polar cell in Asplanchna Sieboldii has been 
described by Lameere (90) from observations upon the living egg. 
The general features of the process are similar in Asplanchna Herrickii, 
though the finer nuclear phenomena differ from those described by 
Lameere. An account of the finer nuclear phenomena is, however, 
foreign to the purpose of this paper: it is necessary to describe merely 
the general features of the process, especially concerning the place where 
the process occurs, in relation to the later orientation of the embryo. 
As is now well known, but a single polar cell is commonly formed in 
the parthenogenetically developing eggs of the Rotifera. The subject 
has received full discussion, especially by Weismann und Ischikawa (87) 
and Lameere (90). It may be noted that Zelinka (91) observed that in 
a number of eggs of Callidina two polar cells were formed ; whether these 
arose by division of a single one, or whether the two were formed. separ- 
ately from the egg, is not stated. In no case have I obtained any evidence 
09 
indicating the formation of more thau a single polar cell in Asplanchna. 
