312 ILLINOIS STATE ACADEMY OF SCIENCE 



average length was 43 cm. and that the girls averaged 3 

 cm. less than the boys. The average height of boys was 

 45.5 cm., that of girls 41.5 cm. Newborn twins, especi- 

 ally those of the same sex, may be approximately of the 

 same height and weight. 



On the other hand, there may be great disparity in 

 their weights, varying from 200 to 800 grams and, in ex- 

 treme cases, even 1000 grams. Differences, in height are 

 also noted. Sometimes one infant is 2.5 cm. longer than 

 his mate. 



Differences in the development may be accounted for 

 at times on a purely mechanical basis. In some instances 

 the nutrition of the two fetuses is unequal. In one case 

 the umbilical cord may be short and straight, in the 

 other one, long and winding. It is evident that in the 

 former the blood supply would be greater and the nutri- 

 tion would be better. At other times, one placenta is 

 located favorably on the uterine wall, and the other one 

 is attached in an unfavorable position. The greater the 

 respiratory surface of the placenta, the better the fetus 

 develops. In other words, the larger the placenta, the 

 more closely it is approximated to the uterine decidua, 

 and the more favorable are the conditions of the fetus. 



Newman states in his "Physiology of Twinning" that 

 there is a popular impression that in human twins one is 

 usually stronger and more vigorous than the other. 

 Practical experience tends to bear out this impression. 

 Even in identical twins, there is usually a more vigorous 

 twin who is the dominant member of the combination. 

 One twin tends to gain a physiological ascendancy over 

 the other to the slight or very great detriment of the 

 latter. Spaeth found no evidence that the twins of either 

 type had any definite physiological effect upon each 

 other, though he grants as an evidence of inter-influence 

 the condition of situs inversus viscerum. Newman 

 points out the disadvantages of twinning by saying that 

 when two or more fetuses come to occupy the space 

 usually filled by one, the twins, whether of the one egg 

 or two egg type, crowd each other and compete for the 

 common food supply. In the case of two egg twins, the 

 competition is for placental surface. 



