102 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 proanmiotic 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 

 AYidakowich 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 



