XI, B, 3 Ruth: Development of Twins HI 



four-, six-, or eight-cell stage. He observed as many as eight 

 divisions of one egg. In no case, however, was a complete em- 

 bryo formed. The development was usually arrested at the 

 gastrula stage, but sometimes went as far as the first gill-slit 

 stage. The size of the embryos that developed from the two-, 

 four-, six-, or eight-cell stage always retained a definite relation 

 to the size of the normal embryo — ^that is, if the ovum separated 

 in the two-cell stages, each embryo was approximately one half 

 the size of the normal embryo. When triplets were formed, 

 one was as large as the other two. If a cell became separated 

 at the eight-cell stage, the embryo was about one eighth the size 

 of a normal embryo. Wilson observed that, when the cells failed 

 to separate completely from each other, double, triple, and quad- 

 ruple monsters were formed. 



The work of Newman and Patterson (9) on the development of 

 the nine-banded armadillos undoubtedly explains the occurrence 

 of identical twins. The unusual feature about the development 

 of the armadillo is the fact that in one litter the embryos are 

 always of the same sex — either all males or all females. New- 

 man and Patterson at first believed that the embryos probably 

 developed from one of the blastomeres of the four-cell stage, 

 and that each embryo could, therefore, be looked upon as a lineal 

 descendant of a blastomere. In later investigations, however, 

 they have shown that the blastomeres do not divide at the four- 

 cell stage, but that the young develop rather as a product of 

 agamogensis, belonging to the general category of budding. This 

 budding takes place after the formation of the blastodermic 

 vesicle. In the armadillo, then, we have normally four embryos 

 developing from one ovum. 



Werber,(li) in a recent article, has shown that, if fertilized 

 Fundulus eggs are treated with a very weak solution of butyric 

 acid or acetone, a small percentage of the ova will form joined 

 twins while the others will develop into various kinds of mon- 

 sters, some with a reduplication of parts and others that show 

 arrested development of parts. 



MATERIAL 



In a series of approximately five hundred duck eggs that were 

 incubated for our embryological material, four sets of twins 

 were found. This gives a higher percentage than has been 

 reported by other investigators who have studied chick and 

 duck twins. One set of twins had developed to the three- 

 somite stage; the other three sets, respectively, to the 9-, 11-, 

 and 12-somite stage. In only one set are the embryos com- 



