136 
beneath the equator, moves backwards over a considerable 
distance, while it becomes deeper only quite gradually and 
extends laterally into a crescent. When it finally closes to 
a ring it has certainly already travelled over a distance of 
some 60°. The crescent and the ring into which it passes 
have here only a very small diameter which I do not believe 
to be more than 30°. During its contraction the dorsal lip 
moves considerably faster than the ventral one and after 
the blastopore has reached the slit-like stage the backward 
movement of this slit with regard to the animal pole 
still continues. Thus the dorsal rudiment of the embryo 
covers here considerably more than 180°, the ventral side 
being much shorter. 
Interpretation of the results. — What conclusions may 
now be drawn from the facts recorded and how are these 
to be interpreted? In the first place we may state that, 
since more than half of the base of the embryo is situated 
in front of the place where the dorsal blastopore lip first 
appears, there can be no question about the whole embryo 
being formed by concrescence of the lateral blastopore 
borders. For the assumption that concrescence would play 
a more or less important rôle in the closure of the blasto- 
pore which gives rise to the posterior lesser half of the 
embryo, there is not the slightest evidence. It is quite 
true that in the amphibian egg a fine median line is often 
seen running from the blastopore forward, which strongly 
suggests a concrescence suture. Only, as ROBINSON and 
ASSHETON (1891) remark, this line continues to the fore- 
end of the cerebral plate where the blastopore has never 
been! 1 shall now propose the explanation which seems 
to me to follow from my theory. Two circumstances give 
a peculiar character to the gastrulation of Vertebrates: 
1. the white area indicating the future endoderm is situated 
not opposite the animal pole, as we have good reason 
to assume must have been the case in radiate anceS- 
tors, but shifted more to the future dorsal side, _ 
2. the contraction of the blastopore border occurs In à 
caudad eccentric direction, so that the closure finally 
takes place nearly opposite the animal pole. 
Comparison with Annelids. — As regards the former phe- 
nomenon, the displacement of the white endoderm area IS 
fairly considerable, as shown e.g. by fig. 35. This same 
displacement of the endoderm area, here to the ventral 
