TWINNING IN BIRDS 8i 



double-monster chick (Fig. 38) in which there is appar- 

 ently a fusion in the head region. The two bodies are 

 very symmetrically placed and practically in the same 

 developmental stage (about seven somites). As com- 

 pared with normal single embryos of this age we note 



A t 



iv^^i;i:---. 



Fig. 38. — A practically symmetrical pair of secondarily fused chick 

 twins, resulting from a collision at a tangent instead of head-on as in 

 Fig. 37. It is such cases as this that have convinced some writers that 

 true katadidymi are the result of the symmetrical fusion of separate 

 embryonic axes. (Original.) 



decided differences. Though the brains of the two 

 components are fairly symmetrical, there is obvious 

 crumpling of both. No part of a. normal single head 

 could possibly arise from such a condition. No omphalo- 

 mesenteric veins are present on either component, while 

 the anterior vitelHne vein of one embryo has developed 

 to one side and that of the other to the other side. This 

 condition also could never so regulate itself as to give 



