XI, B, 3 Ruth: Development of Twins 115 



dillos. The third class includes all those twins that are joined — 

 probably due to an incomplete separation of the early blastomeres 

 or of the embryo-forming substance at some later stage. To 

 this type belong the Siamese twins, who were joined to each 

 other along the ventral part of their abdomen. Wilder (12) has 

 shown that identical and joined twins belong to the same cate- 

 gory. In a series of schematic drawings he has shown that in 

 the Janus type the twins may be attached to each other from 

 their head to the lower part of the abdomen, or they may be 

 separated from above downward until the attachment grows less 

 and less and finally each embryo has a separate umbilical cord, 

 thus resulting in the formation of true identical twins. 



It seems evident, therefore, that the factor which is present in 

 the production of identical twins must likewise be present in the 

 formation of joined twins. We can conceive of a determiner 

 that regulates the moulding of the body into a certain form and 

 type; further, that this determiner divides equally in the first 

 stages of segmentation — in the nine-banded armadillos as far 

 as the blastula stage when the division of the fertilized ovum 

 first begins and a set of identical quadruplets develops. 



In the four sets of duck twins, three sets were joined while 

 in the remaining one the twins were separated. To say that 

 the last set belongs to the first class or to the second class would 

 be mere speculation, as it may answer the description of either 

 type. Not until further experiments have been carried out on 

 the development of twins in amniotes can we say positively that 

 a set of twins was produced by an extrinsic or an intrinsic 

 modification of the environment. The other three sets of joined 

 twins undoubtedly developed from a single, fertilized ovum, with 

 only a partial separation of the early blastomeres, or at a 

 slightly later stage in the process of development. 



BIBLIOGRAPHY 



(1) Mall. Origin of human monsters. Journ. Morphol. (1908), 19, No. 1. 

 (2)-(5) Cited by Mall in Origin of human monsters. 



(6) Driesch. Entwicklungsmechanische Studien. Zeitschr. f. wiss. Zool. 



(1893), 55, 1. 



(7) LoEB. Uber eine einfache Methode zwei oder mehr zusammengewach- 



sene Embryonen aus einem Ei hervorzubringen. Arch. f. d. ges. 

 Phys. (1894), 55, 525. 



(8) Wilson. Amphioxus and the mosaic theory of development. Journ. 



Morphol. (1893), 8, 579. 



(9) Newman and Patterson. A case of normal identical quadruplets in 



nine-banded armadillos and its bearing on the problems of identical 

 twins and sex determination. Biol. Bull. (1909), 17, 181. 



