No. 1.— The Early Development of Asplanchna Herrickii de 
Guerne. A Contribution to Developmental Mechanics] By 
HERBERT S. JENNINGS. 
CONTENTS. 
PAGR PAGE 
Introduction . 2| 2. Maturation . » " 19 
Part First. Developmental Mechantos . 4| 3. Orientation of the Developing I 1 
I. Statement of Problemm 4 biyot Wo 1608 og 14 
1. Clea vage boss uw ae eden, eerie en EH 16 
A. Direction of Cleavage «is ric Nomenclature . . . + + + 16 
(1) Berthold’s principle of least s sur- First Cleavage . « « 18 
faces . . 4 Second ‘ Diu DESTIN M 
(2) Hertwig's lew of the apial in Third: ut 1 ood e o 2M 
the longest axis of the proto- Fourth“ „ 
ne mass RR, Bin S 33 
(3) Braem's theory of separation in Sixth S . 95 
the direction of least resistance 5 Seventh and Later Cleavages 45 
(4) Roux's theory of a compromise The Ectoderm. x « + + + 8 
between the tendency imma- The Entoderm . » . 54 
nent in the nucleus and the III. Discussion of the Bearing of the 
tendency due to the form of Observations on the Problems . 58 
the protoplasmic mass . + 6| 1. Cleavage . » - H onov 88 
(5) Heidenhain's problem of a defi- A. Direction of Cleay age 58 
nite angle of rotation 6 (1) Berthold's theory of least siii 58 
(6) Sachs’s law of the right- Jangted (2) Hertwig's law of the nn in 
arrangement of cell walls. . 7 the longaxis . i~i 60 
(7) Rauber's theory of inter-attrac- (3) Braem's theory of least alben 69 
tion of asters . ERA. ° (4) Roux’s theory . . . 70 
(8) Braem’s principle of equi re- (5) Heidenhain's problem ot a defi- 
sistance at the two ends of the nite angle of rotation . . . 72 
spindle " 7 (6) Sachs's law of the right-angled 
B. Equality or Tosanality of Cleav age 7 arrangement of cell walls .. 73 
C. Rate of Cleavage . 1 (T) Rauber's theory of the inter- 
D. Differentiation during Cleav age. 9 attraction of asters . . . . 74 
2. Later Developmental Processes . 9 (8) Braem's principle of equal re- 
II. Descriptive Portion . . 10 sistance at the two ends of the 
1. Form and Structure of the E gg . 10 Some « 1 € xoa co Ae 
1 Contributions from the Zoölogical Laboratory of the Museum of Comparative 
Zoölogy 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 [1], 1896. 
VOL, XXX. — NO. 1. 1 
