1273 
this has occurred, but in that in Anurans the tube-formation, i.e. 
the closure of the medullary folds, occurs a little before the anus 
is reached, in Urodelans, Dipnoans and Cyclostomes only after this 
has occurred. And this, only graduated difference evidently again 
depends on the circumstance that in Urodelans the activity of the 
periporal growing zone is stronger than in Anurans, the activity of 
the perianal on the contrary weaker than in the latter. This manifests 
itself, as stated above, in the medullary plate in Urodelans being 
developed very strongly, the ventral side very little in comparison 
with the Anurans. The same holds for Dipnoans and Cyclostomes. 
Now, as we have seen, the perianal growing zone acts mainly 
ventrally and on both sides of the (future) anus, for the simple 
reason, that, as long as the medullary plate is open, the future 
trunk ectoderm also lies only ventrally and on both sides of the 
ege. But in front of the (future) anus too, there seems to be some 
feeble activity, directed against the ventral blastopore lip, which 
accordingly is developed more strongly where the perianal growing 
zone is most active (Anurans, fig. 1, plate), less so, where the 
perianal growing zone is less active (Urodelans ete., fig. 5). 
Now the action of this dorsal part of the perianal growing zone 
is opposed by the periporal growing zone, which pushes the blasto- 
pore backwards. And it is no doubt due to the relative strength 
of the two growing zones that in Urodelans the blastopore is pushed 
back to the anus before the tube-formation'), in Anurans on the 
contrary it does not reach it till after the tube-formation. I hope 
that the brevity with which I am obliged to express myself will not 
militate too strongly against the clarity of this exposition. A more 
explicit review will doubtless be published later. 
While I feel that the application of my theory has thus thrown 
light on a number of obscure problems, the facts and results 
recorded above afford yet further support to my theory of no 
inconsiderable value. 
LITERATURE. 
ASSHETON, R., 1909, Professor Huprecut’s Paper on the Early Ontogenetic 
Phenomena in Mammals. Quart. Journ. Mier. Se. N. S. Vol, 54. 
BALFOUR, F. M., 1881, A Treatise on Comparative Embryology. 
Bres, E. J., 1905, The Life-History of Xenopus laevis. Trans. R. Soc. Edinb. 
Vol. XXXXI. 
1) The beginning of which is determined again by the end of gastrulation, just 
as in Protostomia the stomodaeal tube is formed directly after gastrulation. In 
Selachians, where the accomplishment of the gastrulation is so much retarded by 
the great yolk-richness, urogenesis actually sets in before the tube formation, the 
neurenteri¢ canal thus originally being an open groove (sulcus neurentericus), 
