SUMMARY. 287 



240°; and embryo A. consists of a mother and a daughter half; B. of a daughter 

 and a grand-daughter half, and C. of a granddaughter and a mother half. 



Some of the conditions that are actually realized are shown in Fig. 189. 

 In all these cases, median fusion and degeneration accompanies, or follows the 

 formation of triplets. 



In Fig. 189, A, embryo A. is practically normal; embryo B. has undergone 

 median fusion and degeneration, almost resulting in transverse fission at the 

 fourth segment. The abdomen and last two thoracic appendages are practically 

 normal, while the anterior part of the thorax and cephalic lobes has disappeared, 

 except one pair of fused appendages. In embryo C. median fusion and degen- 

 eration have obliterated all but the abdomen and the last pair of fused thoracic 

 appendages. 



In another triplet (Fig. 189, B), embryo A has undergone median fusion and 

 degeneration, forming a good example of an hour-glass embryo. The same proc- 

 ess has affected embryo B, entirely obliterating the cephalic lobes and the anterior 

 portion of the thorax; the dorsal organs, however, are not quite fused in the median 

 line. But this has taken place in embryo C, and in other respects the degeneration 

 is carried farther than in B. 



In still another triplet (Fig^ 189, C), all three embryos are reduced by aniero- 

 posterior fusion and degeneration nearly to the same level; each one retains an 

 unpaired sixth thoracic appendage, a remnant of the abdomen, and an unpaired 

 dorsal organ. 



Multiple embryos, therefore, are formed by the appearance of new halves 

 between the old, the various organs being formed in a definite and orderly manner. 

 After a time degeneration begins, the organs disappearing by a method and in an 

 order that are the reverse of those in which they were generated. 



VI. Summary and Conclusion. 



1. The variations here described are primarily due to structural variations 

 resident in the ovum, and not to differences in the environment. 



2. There is a great difference in the growth rate, under apparently the same 

 conditions. 



3. There is a great difference in size, some embryos of the same stage being 

 much larger than others. 



4. Certain organs or regions of the body may be entirely absent, and are not 

 subsequently restored. 



5. When organs disappear it is usually by median fusion and degeneration, 

 in the reverse order of their age and speciahzation. 



6. Multiple embryos are produced by the formation of new halves between 

 the old, the process beginning at the head end. The new organs are produced in 

 the reverse order to that in which they are formed by normal apical growth, that is, 

 the lateral ones before the median ones. The arrangement is also reversed, the 



