121 
of the egg, it follows that the blastopore has also originally 
nearly the same diameter, and in the figures given by 
ROUX (1888, p. 698) and HERTWIG (1892) for the eggs 
showing the Spina bifida-phenomenon, this indeed proves 
to be often the case, the blastopore-border extending round 
the equator of the egg. In normal eggs, however, the dia- 
meter of the blastopore, even if we measure the distance 
between the place where the dorsal rim first appears, and 
that where the ventral lip appears, never reaches 90°. As 
we shall see, the pricking experiments made by different 
authors and by myself lead to conclusions quite different 
from those of HERTWIG and are in no way to be recon- 
ciled with the latter. Thus, Il think EYCLESHYMER (1895, 
p. 388) is quite right in questioning the entire evidence 
adduced from pathological forms and so probably is WILSON 
(1900) when he suggests that the spina bifida-pheno- 
menon might have to be interpreted by a kind of rupture 
or rolling in of the dorsal lip. That something cf the kind 
occurs is rendered probable by observations on eggs with 
a tendency to spina bifida which 1 shall mention further 
on (cf. p. 133). 
As we shall see later, the study of the movement of the 
blastopore border witn the aid of artificial marks proves, 
that there can be no question about the whole dorsal side 
of the embryo being formed by concrescencc since the 
greater part lies in front of the place where the dorsal 
border of the blastopore first appears, In Amphioxus also 
several authors (LWOFF, 1894, GOETTE, 1895, SOBOTTA, 
1897, GARBOWSKI, 1898, KLAATSCH, 1898, MACBRIDE, 1898) 
have emphasized that no evidence in favour of the con- 
crescence theory can be adduced from the observed facts. 
[ shall not deny that concrescence ever plays a rôle in 
the closure of the blastopore, especially not in the case 
of yolk-laden eggs, but this is only a secondary pheno- 
menon to which no primary phylogenetic significance can 
be attributed., Also the formation of the primitive streak in 
Amniotes e. g. must be explained by concrescence of part 
of the blastopore border which has lost its original 
character. 
Invagination of ectoderm cells? — LWOFF (1894) has 
een the principal advocate of another tendency in inter- 
preting the process of gastrulation in Chordates. As stated 
above, the entoderm-cells, e. g. in Amphibians, are of unequal 
