GASTRULATION AND EARLIEST DEVELOPMENT 159 



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 b), as 

 shown clearly by fig. 33. 



More 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 expe- 

 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. I 

 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 thepointof 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 transverse 



