Kil ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



plate cells. A 74 and A rs , while the lower yolk-laden part becomes the chorda cells, 

 A 73 and A 77 . The chorda-neural substances arc thus contained in the same cells 

 until the sixth cleavage, though their substances are distinct at a much earlier 

 period. Still other instances might be cited to show that the planes of local- 

 ization and the planes of cleavage do not always coincide. This is in part due to the 

 fact that the boundaries of the different kinds of germinal material, e.g.. the yellow 

 protoplasm of the Cynthia egg, are not as sharp as are the boundaries of the cells, 

 and consequently the cleavage furrows cannot precisely separate different kinds of 

 germinal material. Nevertheless the cleavage planes are, under normal conditions, 

 constant in position and character and bear a constant relation to the planes of dif- 

 ferentiation. But that this relationship is not a casual one is further indicated by 

 experimental studies on cleavage in which the position of the cleavage furrows may 

 be altered without altering the localization of germinal materials or the typical form 

 of development. Therefore the factors which determine localization and those 

 which determine the form of cleavage are more or less independent. 



All of these tacts speak unmistakably for the view that localization is more 

 fundamental than cleavage as Whitman (1893) has so ably maintained, and that 

 such correspondence as may exist between the two is of secondary origin and of 

 minor importance. Nevertheless the extreme constancy of cleavage forms shows 

 that we have here a phenomenon, which if of secondary importance to germinal 

 localization, is still of real significance. I have shown that in Crepidula the 

 cleavage is a localizing factor, though secondary in importance to protoplasmic 

 movement, and it seems probable that Wilson (1003) is right when he argues that 

 the relative isolation produced by cleavage gives opportunity for the increase of 

 any initial differences which may exist in the cells at the time of their formation. 



Finally it must be concluded as a result of both observation and experiment 

 that the type of cleavage is less constant and less fundamental than the type of 

 localization, but that cleavage may itself be a factor in the progressive specification 

 of cells {cf. Wilson, Lillie, Conklin, et al.). 



E. Types of Germinal Localization ; Evolution of Types. 



The wonderful resemblances in the germinal localization of annelids and 

 mollusks, as shown especially in the cleavage, have been repeatedly commented 

 upon. Furthermore this localization is for shadowed in the egg before cleavage 

 begins, and this suggests the inquiry as to whether the resemblances between types 

 of localization grow closer as one approaches the ovocyte, and whether the man- 

 ner as well as the results of localization are comparable in the different types. At 

 present our knowledge of the localization in these earliest stages of development 

 is very incomplete, and a comparison can be drawn only between annelids, mol- 

 lusks, ctenophores, echinoderms, ascidians and possibly nemerteans and nematodes. 



In most of these phyla a peripheral layer of protoplasm is present before 

 maturation, which after maturation and fertilization collects at one or both poles 

 of the egg; also with the possible exception of the ctenophores and nematodes, 



