346 G. CARL HUBER 
Selenka, it would seem, in part at least, interpreted correctly 
the development of the mesoderm in the rat, although a stage 
showing its anlage was not observed. Duval believes that the 
mesoderm has origin from a thickened part of the entoderm, 
probably in the region of the anterior portion of the future em- 
bryonic area; the primitive streak was not recognized. Christi- 
ani’s figures 45 and 47, transverse sections of the egg-cylinder 
from the eighth day, give correctly the relative position of the 
mesoderm with reference to the primitive streak; however, they 
show stages some little time after the anlage of the mesoderm. 
According to Robinson, in the early part of the eighth day the 
cavities of the epiblast (primary embryonic ectoderm) and of 
the trophoblast (extraembryonic ectoderm) meet and fuse to 
form a hollow cylinder, the proamniotic cavity. He states 
that ‘“‘For a time the united cavities of the epiblast and tropho- 
blast increase in size, together with the general growth of the 
ovum, and this increase continues until in the latter part of the 
eighth day the mesoblast appears around the margin of the 
epiblast where it is in apposition with the trophoblast.”’ Robin- 
son was able to differentiate between the primary embryonic 
ectoderm (epiblast) and the extraembryonic ectoderm (tropho- 
blast) and his figure 14 (plate 23-24), though schematic, shows 
that he recognized the positions of the anlage of the mesoderm 
correctly, as also its derivation from the primary embryonic 
ectoderm. The observations of Melissinos, bearing on the an- 
lage of the mesoderm have been critically reviewed by both 
Widakowich and Sobotta, and I am wholly in accord with their 
views when they state that no credence can be given these ob- 
servations since it is clear that Melissinos has confused sagittal 
and frontal sections in such a way as to make his observations 
of no value. According to Melissinos, the mesoderm arises 
from the outer surface of the middle fold. of the egg cylinder, 
in the region of its union with the antimesometrial ectodermal 
fold; it is certain that it does not arise from the part of the egg- 
cylinder that has differentiated from the primary embryonic 
ectoderm; but, if I interpret him correctly, from the extra- 
embryonic portion of the ectoderm. That Melissinos did not 
