DEVELOPMENTAL HAZARDS OF HUMAN TWINS 151 



Complete acardia (holoacardia) occurs only in 

 placenta- type D (in which in addition to villous trans- 

 fusion, there are both arterial and venous anastromoses) 

 in connection with vascular conditions d and e for the 

 favored twin and d, e, f, and g for the injured twin. Hemi- 

 acardia occurs only in placenta-type D under exactly 

 the same conditions as does holoacardia. Various de- 

 grees of macrocardia and microcardia occur frequently in 

 connection with all four placenta-types and in connec- 

 tion with most of the vascular conditions. When one 

 twin of a pair shows microcardia the other shows 

 macrocardia. In Figure 50 is shown a typical placenta 

 and the two hearts of the associated twins, that on the 

 left being microcardiac and that on the right macro- 

 cardiac. Undoubtedly various minor degrees of micro- 

 cardia and macrocardia exist in even supposedly normal 

 one-egg twins, and these conditions may have a perma- 

 nent effect on their vitality. 



MORPHOLOGICAL RESULTS OF ACARDIA 



Naturally we should expect that acardia would be 

 accompanied by other more or less serious consequences; 

 for no pronounced slowing up or deficiency of blood 

 could occur without affecting the developing fetus. In 

 general it may be said that the particular defects in- 

 duced depend more or less directly upon the magni- 

 tude of the departure from normal and the particular 

 type of vascular disturbance which has brought about the 

 acardia. Schatz distinguishes the following well-defined 

 types of acardii on the basis of their general morphology : 



I. Acardii completi, which possess head and trunk, 

 but may be more or less deficient in arms or legs. 



