METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 59 



defective auditory vesicles, defective or absent tails. Werber also 

 noted the same fact which as been mentioned in connection with 

 Kellicott's experiments, namely, that in some cases, the parts 

 which are usually inhibited may alone survive, the rest of the 

 embryo having disappeared. Such isolated parts are the anterior 

 end of the embryo and the eyes. In some cases the only differ- 

 entiated parts of the embryo were a fragment of the brain with 

 an eye attached. 



Similar terata can also be produced by hybridization. Such 

 terata in Fund nl its hybrids were described by Newman ('08, '17) 

 and Loeb ('15). Newman showed that there is a correlation 

 between the rate of development of such hybrids and the degree 

 of abnormality. Those which develop most slowly showed the 

 most pronounced abnormalities. The terata are of the same 

 types as those already described, consisting of defective and in- 

 hibited heads, brains and eyes, defective hearts, shortened bodies, 

 as well as types in which the head and eyes alone are present. 



It is highly significant to note that similar terata can be ob- 

 tained by treatment of the sperm alone. Oppermann ('13) ob- 

 tained them from normal eggs of the salmon fertilized by sperm 

 which had been exposed to radium and mesothorium. The 

 embryos resulting from such fertilizations show all of the typical 

 defects distortions and marked inhibition of the forebrain and 

 eyes and general anterior end of the body, defects or inhibitions 

 of the tail, spina bifida, some abnormality of the myotomes. 

 Embryos were frequently obtained having neither definite heads 

 nor tails, but only trunks. G. and P. Hertwig ('13) treated the 

 sperm of Gobius jozo with methylene blue and methyl green and 

 observed that eggs fertilized by such sperm produce abnormal 

 embryos with defective anterior and posterior ends. 



From this consideration of the literature it is obvious that a 

 large variety of agents and conditions produce the same defects 

 in fish embryos. These defects are primarily concerned with the 

 following parts of the embryo : the forebrain, the head in general, 

 the sense organs, especially the eyes, the heart and circulatory 

 system, the tail. 



The explanations of these defects have been almost as numer- 



