No. 2.— The Mesoderm in Teleosts: especially its Share in the 
Formation of the Pectoral Fin. By E. R. Boyer} 
TABLE OF CONTENTS. 
Pace PAGE 
FsIntrodneuonys -0 5 «seni | OL Mesoderm in the Pec- 
II. Preliminary Account . . . 93 toral Region . .. . 104 
(a) The Material . . . . . 93} V. Intermediate Cell-mass. . . 110 
(b) Technique employed . . 95 (a) Review of the Literature 110 
(c) Variation in the Develop- (b) The Origin of the ‘“ In- 
ment of Fundulus .. 94 termediate Cell-mass ” 
III. Summary of the Literature on in Hundulus). )- 320.) bit 
the Development of Paired (c) Concerning the Fate 
Winesin Hashes) xa.) 1s) et OD of the ‘“ Intermediate 
(a)uTReleosts:suy.8'2 ys Sain es) 295 Cell-mass<?” Getsrn, <2 db 
(b) Elasmobranchs, etc.. . . 99 (d) Comparative Review . 120 
IV. The Axial and Parietal Meso- VI. Origin of the Pectoral Fin . 122 
dermyamee oy ts ere we LOZ (a) Proliferation of Somato- 
(a) Formation of the Primitive pleure: .) 2 a. 7c) eh epee 
Layers, and the Differ- (4) Contribution of Myotomic 
entiation of Protoverte- Elements to Pectoral 
bree and Lateral Plates. 102 Plates) cps.) ae or bod ee 
(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 
I DUOPTAD IY) Mi wate cess c: leh ge), “s) VeONSIAR A bec ORR as oe ie en ho en 
PISAnALOM OL RISHEES 6) ete ei oe eee se nt) ede) oe. pee 
I. InrropuctTion. 
BaLFour was the first to study the ontogeny of the paired fins in 
Elasmobranchs 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 Zodlogical Laboratory of the Museum of Comparative 
Zovlogy, under the direction of E. L. Mark, No. XXXI. 
VOL. XXIII. —NO. 2. 7 
