374 The American Xutumlis 



[May. 



heads,"absent medullary folds, etc. After the first cleavage 

 the second plane often passes through the wound and after 

 the second.'cleavage one plane sometimes changed so as to 

 pass through the wound now made. The resulting embryos 

 often have circumscribed areas of deficient development. 

 When injured after the equatorial plane was formed normal 

 tadpoles were found amidst abnormal ones. 



Operations after the fourth, fifth and sixth planes have 

 appeared show that injury to the black pole produces defects 

 in the region of the medullary folds. Blastuke injured nine- 

 teen hours after fertilization show fewer cases of circumscribed 

 defects. In addition it is to be noted that the exuded part 

 of the egg may undergo by itself, a sort of cleavage resulting 

 in the formation of a mass of numerous cells. 



It thus appears that all the material of an egg is not neces- 

 sary to form a normally shaped embryo: that rough me- 

 chanical disturbance of the egg material does not produce 

 complete irregularity in the subsequent arrangements of 

 organs: that circumscribed injuries often produce circum- 

 scribed defects and that about the same eilect results what- 

 ever stage of cleavage is injured. 



The author, however, does not know why the defects are 

 sometimes absent, nor can he produce the same defect at 

 pleasure, in different experiments. Yet the methods of injur- 

 ing definite areas of the egg is made use of in referring the 

 cleavage planes to the axes of the subsequent embryo. Thus 

 when eggs in the two celled stage have the needle thrust into 

 them in the black pole, at the uppermost part of the border 

 between black and white, the egg being naturally inclined, a 

 circumscribed defect appears posterior to the middle of the 

 medullary folds, whence we see that the posterior part of the 

 medullary folds were formed over what was the white pole 

 and the third plane divides head from tail substance. 



But this question of axes and position of embryo will he 

 dealt with later on in connection with other experiments. 



The gastrula also was injured by the needle, but with con- 

 flicting results as to the circmnseribed nature of the resulting 

 defects. Deep cuts made into tin- ^inl, vield the interesting 



