MESODERM AND THE TAIL IN PETROMVZON. 



59' 



mcscuchymatous cells, which migrate into the interior of the epidermal 

 fold of the caudal fin and are transformed into the horny fin-rays and into 

 the connective tissue and muscles of the fin. In short, the tissue is 

 altogether used up in forming the mesodermal organs. In the third place 

 and lastly, the similar structure is formed in front of the normal 

 mesodermic somites and is designated as the first somite, because it is 

 found in segmental correspondence with the foremost ganglion, the 

 ophthalmic, and with the premandibular vascular arch which represents 

 the foremost vasomere. 



On the thirtieth to thirty-second day, the tail is finished with its- 

 development. The subsequent growth is carried on by differentiation and- 

 growth of the chorda, the mesodermic somites, and the medullary cord in 

 the tail 



From the history of the peristomal mesoderm above given, it is 

 apparently to be suggested that the tail can not morphologically be 

 distinguished from the remaining portion of the body lying in front of it. 

 The differentiation of the enteroderm reveals, however, as it seems to me,, 

 the true nature of the tail. 



Early in the development of the tail, the enteroderm forms a conical 

 tube of epithelium, the postanal gut, which runs lengthwise beneath the 

 chorda, and is separated from the latter by the first rudiment of the caudal 

 arter)-. By its proximal end the postanal gut opens into the anal gut and 

 at its distal end it stands in connection with the indifferent tissue of the tail 

 bud. The highest development of the postanal gut is attained at about the 

 sixteenth day, and thereafter it degenerates in the following way. The 

 capillary vessels which stand in connection with the fore-runners of the 

 caudal artery and vein running respectively above and below the postanal 

 gut, develop at first around this gut and are later converted into a thick 

 network which, absorbing the gut, occupies its position. 



What interests us is that the anal gut which permanently retains its 

 downward bending and is in these later stages almost vertical in position, 

 communicates with the postanal gut by the whole height of its hind wall 



