METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 23 



the primitive streak. Similar dwarfing accompanied by other 

 abnormalities resulted when the unvarnished circle was over the 

 anterior or posterior part of the blastoderm. Windle ('93) 

 applied varnish to different parts of the egg. Among the 

 abnormalities regularly obtained were: defects of the area 

 vasculosa, absence of the embryo, deformed embryos. Here 

 again the account is too lacking in detail for interpretation. 

 Mitrophanow ('oo) also tried the effect of partial shellacking of 

 the egg, combining this procedure, however, with heightened 

 temperature. Upon shellacking that half of the egg containing 

 the anterior half of the embryo and incubating at supernormal 

 temperature (about 45 C.) Mitrophanow noted a general inhi- 

 bition of development of the anterior part of the primitive streak 

 and of the head process with broadened and thickened posterior 

 ends. Mitrophanow regards such posterior ends as overde- 

 veloped but it seems to me that they may represent inhibitions. 

 It appears that if the posterior center of activity is somewhat 

 inhibited it tends to pile up into masses of cells instead of 

 elongating in the posterior direction. In one case, which was in 

 the early medullary fold stage when opened, the primitive knot 

 region was found greatly inhibited, regions anterior and posterior 

 to this being more or less normal. Shellacking the posterior half 

 of the egg did not yield any definite modifications of development 

 except some inhibition of the posterior end. Mitrophanow was 

 of the general opinion that the posterior end of the early embryo 

 is most susceptible to modification. The most extensive experi- 

 ment on the production of abnormalities in birds by an altered 

 gaseous environment is that of Riddle ('23). Unfortunately the 

 data furnished by Riddle as to the modifications induced are 

 very meager. Further the embryos were examined mostly at 

 late stages or at hatching and many of the most abnormal 

 embryos were thus probably missed, having died at early stages. 

 Riddle records several dove and pigeon embryos with defective 

 or absent eyes, small heads, and other head defects as a result of 

 exposure to air reduced in oxygen content (u to 13 per cent, 

 oxygen) during about 24 hours in the first three days of incu- 

 bation. Excess oxygen with or without increased carbon dioxide 

 content yielded similar abnormalities. Riddle states that nearly 



