JENXINGS: DEVELOPMENT OF ASPLANCHNA HEKRICKII. 11 



that point. The walls of the oviduct invest the egg closely, and gener- 

 ally cannot be removed, so that a thin membrane, if present, could not 

 be detected. In most cases where it was possible to remove the walls 

 of the oviduct entirely, no membrane could be seen either in sections or 

 whole preparations, the egg appearing to be naked. In a few cases, 

 however, in which cleavage had recently taken place and the cleavage 

 furrows were marked, it could be observed that the smooth outline of 

 the egg was preserved even above the cleavage furrow, either by means 

 of a membrane continuing across the furrow, or, wnat seemed from the 

 appearance of the preparations more probable, owing to the presence in 

 the furrows of a fluid mass, perhaps exuded from the egg. Such a case 

 is shown in Plate 1, Fig. 4. On other grounds, however, it seems pos- 

 sible that an extremely delicate membrane is present. Lameere ('90) 

 states that in Asplanchna Sieboldii, which is likewise viviparous, it was 

 possible to observe definitely a very delicate membrane surrounding the 

 egg, especially at the time of the formation of the polar cell. 



This question of the presence or absence of an egg membrane is of 

 importance from a mechanical standpoint, owing to its bearing upon the 

 question as to what preserves the ellipsoidal form of the egg. The form 

 is retained throughout all the early developmental processes; cleaving 

 cells do not project above the general surface of the egg, nor do the 

 products of cleavage become spherical, touching at a few points only, as 

 is common in the MoUusca and other groups. This retention of the 

 ellipsoidal shape by the egg compels the cleaving cells to take various 

 peculiar forms, which allow of a direct test of some of the theories of 

 cell division above stated. It is also a most important factor in the 

 process of gastralation, so that it becomes of great interest to discover 

 what it is that pi'eserves this form. 



It is evident that surftice tension would tend to produce a spherical 

 rather than an ellipsoidal form. Roux ('95) has recently proved that 

 blastomeres have a direct attraction for each other ; but an equal 

 attraction throughout the mass would produce a spherical form, and an 

 unequal attraction, such as would produce a regularly ellipsoidal form, 

 is veiy difficult to conceive of, especially as this attraction would have 

 to vary regularly with the shifting of the contents of the egg. A mem- 

 brane of equal elasticity in all parts would likewise result in the produc- 

 tion of a splierical form. The only direct mechanical flxctor that seems 

 capable of explaining the continued ellipsoidal form is the presence of a 

 non-elastic membrane of the exact size and shape of the egg. But 

 during the later development the embryo enlarges and changes its form ; 



