110 
remaining parallel to itself and perpendicular to the main 
axis of the egg. Approaching the opposite pole it finally 
contracts to a ring, the so-called yolk-blastopore, nearly 
diametrically opposite the place where, at about the same 
time, the rudiment of the head is formed. This appears 
at one of the extremities of the oblong egg, while the 
blastopore closes at the opposite end. From this it is 
evident that the head and the fore brain are formed in the 
neighbourhood of the animal pole (DELSMAN, 1913 6), as 
shown clearly by fig. 33. 
ore conclusive evidence is yielded by pricking expe- 
riments. SUMNER (1904) operated on the egg of some North- 
American species of the Teleostean genus Fundulus. When 
he pricked in the centre of the still small germinal disc, 
which later appears to extend here also in a concentric 
way over the whole egg, the mark was found afterwards 
exactly in front of the anterior end of the rudiment of the 
embryo, just where we might expect it from our theoretical 
considerations. 
Amphibian eggs. — Similar were the results of pricking expé- 
riments on several kinds of Amphibian eggs. EYCLESHYMER 
(1895, 1898), operating on the eggs of the axolotl and the 
American frog Acris, also rediscovered the mark either just in 
front of, or upon, or just behind the transverse brain-fold. 
myself (1916) obtained the same result with the eggs of Rana 
fusca, Rana esculenta and Amblystoma tigrinum. The eggS 
when in the 4- or 8-celled stage, after having been freed 
from the surrounding jelly, were placed in a small glass- 
scale with water and cotton wool and under slight micros- 
copical enlargement were pricked with the point of the quill 
of a hedge-hog in such a way that only a very trifling 
wound was made. For with a somewhat more serious 
lesion a considerable extraovate protrudes at once, the size 
of which increases during the subsequent cleavage and 
which results in abnormal development. In working with the - 
eggs of Amblystoma, where the protoplasm has a very fluid 
consistency and at a little lesion already protrudes in large 
quantity, it appeared necessary first to sharpen the hedge- 
hog quill on a smooth file. In this manner a very fine 
point could be obtained. The eggs were marked in the 
four- or eight-celled stage at the crossing-point of the first 
two cleavage-furrows and the mark was found again In 
all three kinds of eggs on, or just in front of, the transversé 
