380 Julia B. Platt 
cells of mesodermie origin, and v. KÖLLIKER (’S4) considers the 
question of their origin in any case enigmatical. 
HEIDENHAIN ('93), however, calls attention to the fact that the 
muscle cells in question are connected, in the Amphibia, with the 
epidermis by intercellular strands of protoplasm similar to those 
found between the epidermis cells, and finds in this organic continuity 
evidence that the muscle cells belong genetically to the outer germ- 
layer. v. KÖLLIKER (’84) moreover, adds that the same difficulty 
occurs in connection with the endoderm, since here also smooth muscle 
fibres have been found immediately on the endodermal epithelium 
of the lung in the Mammalian embryo, as testified by Srrepa (78) 
and himself (79). 
The vascular system is claimed for the mesoderm. 
Yet GoErTE tells us that in the Amphibia (’75) and in Pe- 
tromyzon (90), the first blood cells come from the endoderm, and 
his observations have been confirmed by Scuwink (91). Houssay 
(93) also affirms that the endoderm takes part in the formation of 
the vascular system in Axolotl, while Horrmann (93) even claims 
an endodermic origin for the vascular system in the Selachii’. 
GOETTE is inclined to believe the endodermie origin of the blood 
the more primitive, while ZIEGLER (92, page 28), regarding the 
mesoderm as the primitive source of vascular tissue, calls attention 
to the fact that the blood cells in the Amphibia arise in places, 
where, in the younger embryo, there was a continuous connection 
between mesoderm and endoderm, from which ZIEGLER concludes 
that it is »theoretisch denkbar« that in the course of phylogeny the 
place of origin of the blood was displaced from the mesoderm to 
the endoderm. In the Urodela and young Mammalia, the spleen 
is a source of red blood-corpuscles. The Mammalian spleen is of 
mesodermic origin, but according to MAURER (90), the spleen of 
Amphibia is derived from the endoderm. 
ZIEGLER adds that most zoologists assign to the endothelium of 
the blood vessels the same origin as to the blood corpuscles. The 
endothelium is therefore usually described as mesodermic, but GOETTE 
(loc. cit.) and Scuwink (loc. cit.) derive the endothelium of the heart 
in the Amphibia and in Petromyzon from the endoderm, and 
1 My sections of Acanthias embryos convince me that HOFFMANN was 
unfortunate in the preservation of the material on which this statement is 
based. 
