MESODERM AND THE TAIL IN PETROMYZON. 



53 



is accordingly divided by the invaginated proctodaeum, and each half of it 

 is contained in each wing. The wings are, as I assume, to be compared 

 with the tail lobes of the Selachian embryos. The tail lobes remain for a 

 time separated, as revealed on transverse sections, but they ultimately 

 coalesce in the dorsal median line with each other, and the rudiment of 

 the tail is produced by this process. The coalescence goes on from before 

 backwards. 



The assertion is, therefore, justified, that the slit separating the tail 

 lobes corresponds to the neurenteric canal of other vertebrates, for it 

 stands in direct connection with the dorsal groove in the medullary plate. 

 Accordingly it may further be true, that the neurenteric canal does not 

 occur only at the completion of gastrulation, but it exists during the whole 

 process of gastrulation, being represented by the notch on the dorsal lip of 

 the blastopore. 



It is probably the coalescence of the tail lobes, that led Kyclethymer 

 > (02) and Selys-Longchamps (10-11) to the conclusion that in Petromyson 

 the concrescence occurs only in a small extent of the hindermost portion 

 of the body. The conclusion is, however, not correct, the coalescence in 

 question represents indeed the last phase of the growth of the blastoporic 

 lips, by which the blastopore is closed. And this is the reason why the 

 tail lobes which lie close to each other, are not at once fused together 

 along their whole extent, but the coalescence is carried on from before 

 backwards. In short, by the growth of the dorsal lip the notch or slit is 

 obliterated. After the finished coalescence there is left, therefore, no 

 suture which ought possibly to be marked at least by arrangement of 

 the cells or by some other structural peculiarities, if the tail lobes were 

 actually fused. 



The hindermost portion of the proctodaeum persists as the permanent 

 anus, while the remainder is closed from the exterior by re-fusion of the 

 edge of the slit. The very short proctodaeum is, on one hand, com- 

 municated with the hind end of the enteric canal and, on the other, it 

 • opens exterior by the anus. At its junction with the enteric canal, the 



