No. 2.— The Mesoderm in Telcosts: especially ats Share in the 
Formation of the Pectoral Fin. By E. R. Bover? 
TABLE OF CONTENTS. 
Pace PAGE 
T Introd uptiorisc sees molas Lana Pl Mesoderm in the Pec- 
IL Preliminary Account . . . 98 torak Region . 0... Ue 
(a) The Material sa 5. «* © V. Intermediate Cell-mass . . . 110 
(b) Technique employed . . 9 (a) Review of the Literature 110 
(c) Variation in the Develop- (b) The Origin of the ** Iv- 
ment of Fundulus . . 94 termediate Cell-mass ” 
Ill. Summary of the Literature on in: Punduli eiea e oe All 
the Development of Paired (c) Concerning the Fate 
Bing in Flöhe . . « ». . 95 of the “ Intermediate 
(C Tu5088 4. $ V4 v — Oslimiass re a NS 
(b) Elasmobranchs, ete. . . . 99 (d) Comparative Review . 120 
IV. The Axial and Parietal Meso- VI. Origin of the Pectoral Fin . 122 
denm» ade doo ees ba OR (a) Proliferation of Somato- 
(a) Formation of the Primitive MOUDE o ue 6 4. a TANE 
Layers, and the Differ- (b) Contribution of Myotomie 
entiation of Protoverte- Elements to Pectoral 
bre and Lateral Plates . 102 Plate. tots «128 
(b) Topography ofthe Embryo 104] VII. Concerning the Relation of 
(c) Differentiation and early the Pectoral Plate and 
Development of the the Ectodermal Fold . . 126 
BU LO TED SE Mundi votado ande oe ceps Tete dis fca s dite t1 p deci 
Danina nonoi RUIN G Bie 2. a ur 2s pac Met d M cin A p xp c NN c LUN 
I. INTRODUCTION. 
JALFOUR was the first to study the ontogeny of the paired fins in 
Klasmobranchs with sufficient fulness to determine the source of the 
musculature. His discoveries practically ended the discussion which 
was aroused by the widely different views concerning the phylogenetic 
origin of the vertebrate limb advocated by Thacher, Mivart, and Balfour 
on the one hand, and by Gegenbaur and Huxley on the other, 
It is a well established rule, that, in the development of the paired 
limbs of Vertebrates, the earliest trace is a lateral ectodermal fold or 
diverticulum, into which subsequently the indifferent mesoderm grows 
1 Contributions from the Zoölogical Laboratory of the Museum of Comparative 
Zoology, under the direction of E. L. Mark, No. XXXI. 
VOL. XXIII. — NO. 2. 7 
