EXPERIMENTAL PRODUCTION OF TWINS 15 



dillo twinning based on the idea that twinning was the 

 direct result of interrupted development, would natu- 

 rally be on the lookout for just this type of result. 



THREE TYPES OF ONE-EGG TWINS 



From the point of view of their mode of origin three 

 distinct types of twins were readily distinguishable: (i) 

 dwarf larvae resulting from the physiological isolation, 

 followed by physical isolation, of the cleavage products 

 of separate blastomeres; (2) double, triple, or multiple 

 individuals resulting from the formation of two or more 

 archentera; (3) ''two-headed" larvae resulting from a 

 dichotomy of the anterior end of the archenteron. 



In addition to these three primary types certain 

 secondary types appeared as the result of fusion of 

 adjacent archentera to form individuals with a A-shaped 

 archenteron, single in front and divided behind (Fig. 5) . 

 Each of these types will be discussed separately. 



I. Dwarf larvae. — These larvae always appeared in 

 parthenogenetic cultures. When the very much belated 

 process of cleavage begins, it proceeds with so little 

 energy that many eggs start the first cleavage and lose 

 their developmental momentum before the two daughter- 

 cells are fully separated. In others the first cleavage 

 completes itself, but the second cleavage takes place in 

 only one of the blastomeres. The belated blastomere 

 either remains permanently undivided or else resumes 

 division at a slower rate than the first. There is obvious 

 physiological discoordination between the two sets of 

 cells thus produced. Each produces a blastula, one less 

 normal than the other, and before long they rupture 

 the membrane and become swimming, half-sized larvae. 



