22 BULLETIN OF THE 
spaces,” are said to move from their original lateral position and unite in 
the median line between the chorda and intestinal canal, which is algo 
true of the intermediate cell-mass. 
When the source and fate of the intermediate cell-mass in Teleosts 
are compared with those of the corresponding structure in other Verte- 
brates, —the ‘intermediate cell-mass” in Elasmobranchs, Lacerta, and 
Birds, —it seems to me that the negation of the homology of these 
structures (Ziegler) is not justifiable; but that, on the other hand, a 
comparative view affords evidence in favor of such homology. The 
variation in the morphology of the intermediate cell-mass in Teleosts, 
Elasmobranchs, Lacertilia, and Birds seems comparatively insignificant 
when we consider its variation in representatives of the same group, 
e.g. the Chick and Duck, and especially when its variation in the 
different regions in the same embryo is considered. When we take 
into account the characteristic peculiarity of the Teleostean embryo, — 
namely, the compact condition of the primitive layers, the embryo being 
pressed down into the relatively large amount of yolk beneath, — a 
canse is found which is apparently adequate to explain this modifica- 
tion in the morphology of structures which are homologous. The points 
of similarity in the origin and fate of these structures in the different 
groups of Vertebrates are strong and significant. 
VI. ORIGIN or THE PECTORAL Fin. 
(a) Proliferation of Somatopleure. — The earliest trace of any modi- 
fication of the mesoderm in the region of the pectoral fin takes place 
opposite the nephrostome (Plate V. Fig. 33, so’plu.), and consists of a 
thickening of the somatopleuric layer. This is observed in an embryo 
of eighty-four hours. In front of this region, opposite the third proto- 
vertebra, the somatopleure consists of a single layer of cells (so’plu., 
Fig. 30), and as such is continuous with the head-mesoderm. At an 
earlier stage, in the region of the first and second protovertebre there is 
a thickening of the somatopleure (Plate IV. Fig. 25, so’plu.), which 
extends forward to the head-mesoderm ; but this is only a temporary 
condition, which exists at the time when the lateral margin of the 
entoderm (en’drm., Fig. 25) begins to fold toward the axis to form 
the intestine (in., Fig. 30). During this period there is apparently a 
crowding of cells toward the proximal region of the somatopleure, which 
later, when the entoderm is well advanced in the process of its infold- 
ing, assumes again the form of a single layer of cells (so’plu., Fig. 30). 
