MESODERM AND THE TAIL IN PETKOMYZON. 



55 



Since the publication of the notable paper by Max Schultze (56), all 

 the previous observers of Pctromyzon maintain the persistence of the 

 blastopore as the permanent anus. Among them Goette (90) alone 

 followed with great caution the development of the hind extremity of the 

 embryo. But overlooked the ventral portion of the peristomal mesoderm 

 as well as the changes of the structure in connection with the proctodaeum 

 and with the rudiment of the tail, he comes again to the conclusion, that 

 the hind corner of the blastopore remains unclosed and constitutes the 

 permanent anus. 



The produced dorsal lip of the anus remains for a very long time 

 externally unchanged, until it begins on the fourteenth day to grow into 

 the conical tail. Very peculiar and interesting is the development under- 

 gone in these ten days by the contents of the first rudiment of the tail, 

 which will for the sake of convenience of reference be called the tail bud. 



The tail bud is formed from the hindermost portion of the peristomal 

 mesoderm which has now completely passed the dorsal surface of the 

 enteric canal and constitutes the hindermost part of the above mentioned 

 median stem of the inner micromeric layer. Anteriorly it passes into the 

 chorda and is, on either side of it, continuous with the gastral mesoderm, 

 while at the posterior end of the rudiment of the tail it is connected with 

 the ectoderm which thickens to the medullary cord, and with the posterior 

 extremity of the enteroderm, which differentiates into the epithelial 

 postanal gut. 



The differentiation, which the rudiment of the tail undergoes during 

 the above mentionded ten days, is almost absolutely confined to its internal 

 structure. Vigorous differentiation of the indifferent tissue of the tail bud is 

 observed in the course of this long period. The products of the differenti- 

 ation do not contribute to the development of the tail, but give rise to 

 trunk somites and add to the length of the chorda in the trunk. The 

 ectoderm and its medullar)- thickening undergo parallel growth with the 

 mesoderm and chorda. 



On the contrary the enteroderm in the mid- and hindgut region, the 



