BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



PAGE 



B. Equality or Ineqiial'tj' of Cleav- 



age Products T-t 



C. Rate of Cleavage 74 



D. Differentiations accompanying 



Cleavage 77 



2. Gastniiation 80 



3. General Considerations .... 83 

 Part Second. Discussion of Matters 



bearing upon the Morphology of 



the Kotifera 87 



1. Previous Knowledge of Asplanchna 

 Herrickil 88 



PASB 



2. Development 89 



A. Maturation 89 



B. Cleavage 94 



C. Summary on Maturation and 



Cleavage in the Rotifera . . 100 

 Part Third. Material and Methods . . 101 

 General Summary 106 



A. Observations 106 



B. Conclusions 108 



Literature Cited HI 



Explanation of Plates 1 17 



INTRODUCTION. 



The following pages contain a study of the early development of an 

 organism, with especial reference to recent theories in regai'd to the laws 

 of cleavage and the relation of cleavage to morphogenesis. 



Many theories and so called laws have been set forth concerning the 

 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 ('94) with regard to the 

 Echinoderm egg, and the recent studies of Ziegler ('95) and zur Stras- 

 sen ('9G) ^ 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 tlie 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 30, 1896, while 

 zur Strassen's ('96) paper was not received here till May 13. 



