IV. i INITIAL STRUCTURE OF THE GERM 179 



Injuries to various parts of the blastoderm and its margin (the 

 germ-ring) prove that it possesses a certain degree of ' isotropy '. 



Thus the entire embryonic region of the posterior margin was 

 destroyed by electrocautery. The adjacent edges closed up and 

 completed the germ-ring; at the posterior point of this a new 

 embryonic shield was formed, in which ectoderm, endoderm, and 

 notochord became differentiated. Or, again, a lateral piece of 

 the germ-ring immediately adjacent to the embryo was removed 

 without preventing the formation of a normal, bilaterally 

 symmetrical embryo, although, as Morgan found, the mesoderm 

 might be deficient posteriorly on the injured side. 



LITERATUEE 



N. KASTSCHENKO. Zur Entwickelungsgeschichte des Selachierembryos, 

 Anat. Anz. iii, 1888. 



F. KOPSCH. Experimentelle Untersuchungen viber die Keirahaut der 

 Salmoniden, Verh. Anat. GeselL, 1896. 



F. KOPSCH. Experimentelle Untersuchungen am Primitivstreifen 

 des Hiihnchens und an Sfcf/Z/wm-Embryonen, Verh. Anat. GeselL, 1898. 



T. H. MORGAN. Experimental Studies on Teleost eggs, Anat. Anz. 

 viii, 1893. 



T. H. MORGAN. The formation of the fish-embryo, Journ. Morph. 

 x, 1895. 



F. B. SUMNER. A study ot early fish-development, experimental and 

 morphological, Arch. Ent. Mech. xvii, 1904. 



4. AMPHIOXUS. 



In this form the blastomeres may be separated by shaking. 

 Their development has been observed by Wilson, whose account 



FIG. 89. A. Normal \ gastrula of Amphioxus. R. Gastrula from a | bias 

 tomero. C. Gastrula from a J blastomere. D. Gastrula of normal size 

 from an egg-fragment. (After Wilson, from Korschelt and Heider.) 



has since been confirmed by Morgan. The isolated blastomeres 

 soon become rounded ; the first division is always transverse to 

 the long axis. 



N 2 



