BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
PAGE PAGE 
B. Equality or Inequality of Cleav- 2, Development 89 
age Products . . . „ „ 74 = Maturation « e + « 89 
C. Rate of Cleavage. . 4 14 3. Cleavage e « v v. 04 
D. Differentiations ^ accompanying " Summary on Maturation and 
Cleavage 6 o e o o T Cleavage in the Rotifera . . 100 
2. Gastrulation . © . . . . . . 80 Part Third. Material and Methods. . 101 
3. General Considerations . . . 88 General Summary , 106 
Part Second. Discussion of Matters A, Observationgs 106 
bearing upon the Morphology of B. Conclusions «e s 1 108 
the Rotifera . . + . 8T | Literature Cited ^ v. e. 111 
1. Previous Knowledge of Asplanchaa Explanation of Plates . .117 
Herſekkll 8 
INTRODUCTION. 
Tue following pages contain a study of the early development of an 
organism, with especial reference to recent theories in regard to the laws 
of cleavage and the relation of cleavage to morphogenesis. 
Many theories and so called laws have been set forth concerning tho 
factors determining the manner and rate of cleavage. These have 
taken the form chiefly of theories in regard to the causes of the direc- 
tion of the spindle, of the equality or inequality in size of the products 
of division, and of the relative rapidity with which the different cleav- 
age cells divide. Yet few attempts have been made to interpret con- 
sistently the cleavage of any given organism with relation to any or all 
of these theories. The sketch of Braem (794) with regard to the 
Echinoderm egg, and the recent studies of Ziegler ('95) and zur Stras- 
sen (^96)! on the Nematode egg, are almost the only works that can be 
cited in which an attempt has been made to show the relation of any 
theory or theories to the series of normal cleavages in any animal. In 
other discussions the theories have been based upon experimental evi- 
dence or upon scattered observations. Yet it is, of course, the normal 
processes for which explanations are desired ; scattered observations may 
be adduced for almost any view. It seems of the greatest importance, 
therefore, to show clearly the exact relation which the theories hitherto 
proposed have to the actual series of cell divisions in the development 
of particular organisms. 
1 In view of the close similarity of some of my conclusions with some of those 
in the more recent (96) of two papers by zur Strassen, it may be proper to state 
that a copy of the present paper, exactly as here published, with the exception of 
some verbal alterations and the addition of a few references, was deposited with 
the Faculty of Arts and Sciences of Harvard University on April 80, 1896, while 
zur Strassen's (90) paper was not received here till May 13. 
