No. 1. — Tlie Early Development of Asplanchna Herric'kii de 

 Guerne. A Contribution to Developmental Mechanics} By 

 Heebekt S. Jennings. 



CONTENTS. 



Introduction 2 



Part First. Developmental Mechanics . 4 

 I. Statement of Problems 4 



1. Cleavage 4 



A. Direction of Cleavage .... 4 



(1) Berthold's principle of least sur- 



faces 4 



(2) Hertwig's law of the spindle in 



the longest axis of the proto- 

 plasmic mass 5 



(3) Braem's theory of separation in 



the direction of least resistance 5 



(4) Eoux's theory of a compromise 



between the tendency imma- 

 nent in the nucleus and the 

 tendency due to the form of 

 the protoplasmic mass ... 6 



(5) Heidenhain's problem of a defi- 



nite angle of rotation ... 6 



(6) Sachs's law of the right-angled 



arrangement of cell walls . . 7 



(7) Rauber's theory of inter-attrac- 



tion of asters 7 



(8) Braem's principle of equal re- 



sistance at the two ends of the 

 spindle 7 



B. Equality or Inequality of Cleavage 7 



C. Rate of Cleavage 8 



D. Differentiation during Cleavage . 9 



2. Later Developmental Processes . 9 

 II. Descriptive Portion 10 



1. Form and Structure of the Egg . 10 



2. Maturation 12 



3. Orientation of the Developing Em- 

 bryo 14 



4. Cleavage 16 



Nomenclature 16 



First Cleavage 18 



Second " 20 



Third " 22 



Fourth " 24 



Fifth " 29 



Sixth " 35 



Seventh and Later Cleavages . 45 



The Ectoderm 45 



The Entoderm 54 



III. Discussion of the Bearing of the 



Observations on the Problems . 58 



1. Cleavage 58 



A. Direction of Cleavage .... 58 



(1) Berthold's theory of least surfaces 58 



(2) Hertwig's law of the spindle in 

 the long axis 60 



(3) Braem's theory of least resistance 69 



(4) Roux's theory 70 



(5) Heidenhain's problem of a defi- 

 nite angle of rotation ... 72 



(6) Sachs's law of the right-angled 

 arrangement of cell walls . . 73 



(7) Rauber's theory of the inter- 

 attraction of asters .... 74 



(8) Braem's principle of equal re- 

 sistance at the two ends of the 

 spindle 74 



1 Contributions from the Zoological Laboratory of the Jluseum of Comparative 

 Zoology at Harvard College, under the direction of E. L. Mark, No. LXX. 



Contribution No. LXIX., under the direction of E. L. Mark, was published 

 under the following title : "Descriptions of Three Species of Sand Fleas (Amphi- 

 pods) collected at Newport, Rhode Island. By Sylvester D. Judd." Proceed. 

 U. S. Nat. Museum, Vol XVIII. No. 1084, pp. 593-603, 11 Figures, August [?], 1896. 



VOL. XXX. — NO. 1. 1 



