12 bulletin: museum of comparative zoology. 



the luembraue caunot 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 chai-acter is, however, 

 very improbable, and it loses all its explaining power when the egg of 

 anotlier rotifer, Melicerta riugens, 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. 



Tlie 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 priodouta Gosse was also examined 

 for comparison with that of Asplanchna Herrickii. The egg of this 

 species is similar throughout to that of Asplanchna Heri'ickii, save that 

 it is smaller. The average dimensions are about 70 fx by 60 /a. 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 Eotifera. 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 

 indicating the formation of more than a single polar cell in Asplanchna. 



