JEXNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKn. 77 



centration des im Theiliuigsstlisk befindlichen Protoplasmas," must be 

 considered — like the correspondiog one in regard to the equality or 

 inequality of cleavage products — as an example of the immature 

 generalizations to which embryology has fallen heir through the acci- 

 dental circumstance that the amphibian egg was for a longtime the chief 

 object for the study of cell division. It seems to be exceedingly diffi- 

 cult to grasp the fact, every day becoming more evident, that because a 

 statement is true for the eggs of some of the lower vertebrates it does 

 not follow that it must be true for the cells of all organisms. 



(4) The rhythm of cleavage has an important relation to the other 

 processes of morphogenesis. If cleavage took place coincidently in all 

 the cells of the egg, the latter still retaining its form, there could 

 apparently be no " rotation " of the cells upon one another, and conse- 

 quently no gastrulation. The tension in all directions would be the 

 same ; none of the cells would be moulded to fit the extension of 

 neigliboriug cells, and all would retain approximately the positions held 

 at the beginning. 



It would perhaps be possible to carry this into detail, and show that 

 the earlier cleavage of the large ventral cell, leaving it in a resting 

 condition, and therefore plastic (as indicated by a comparative study of 

 the forms taken by resting and by dividing cells) when the other cells 

 divide and extend, is directly favorable to gastrulation. 



The question as to the factors determining the time and the equality 

 or inequality of cleavage, it will be seen, does not at present admit of 

 any direct simple answer. Cleavage is a part of the process of morpho- 

 genesis, and its rhythm and other features are related to the nature of 

 the form to be produced. 



D. As to Differentiation accompanying Cleavage. 

 The facts bearing upon this question to be derived from the observa- 

 tion of the early development of Asplanchna are few in number. The 

 principal phenomenon to which attention must be directed is that of the 

 segregation and migration of the cloud of granules, described on pages 

 25, 30, 37, and 54. As will be remembered, a concentration of granules 

 begins in the ventral region of one of the cells at the eight-cell stage 

 (Fig. 7) and becomes more and more marked as successive cleavages 

 take place, till a well defined cloud of very large and distinct spherules 

 occupies the anterior and ventral margin of the cell d^-^ in the stages 

 immediately succeeding the sixteen-cell condition (Fig. 32, Plate 4, and 

 Fig. 48, Plate 6). Then occur the remarkable migrations shown in 



