116 CHARLES R. STOCKARD 



d. An explanation of polyembryony in the armadillo 190 



e. 'Alternation of generations' and twins in vertebrates 197 



6. The structural differences between the two components in connected 



twins and double individuals '. 198 



a. Double individuals with identical or equal-size components 198 



b. Double individuals with unequal components 201 



1. Condition of the larger component 202 



2. Condition of the smaller component 202 



3. The small component and the actual frequency of double or 



twin individuals 207 



4. The small component and certain theories of teratoma 209 



5. Types of defects exhibited by the smaller component 211 



c. The components in double human specimens 212 



7. The double individual with unequal components and an understanding 



of the cause of all monstrous development 218 



8. The double individual with unequal components and an analysis of the 



development of organs in the single individual 228 



a. The growth influence of the apical or primary bud over the secondary 



and potential buds in plants 229 



b. The initial linear growths, subsequent lateral buds, and the inter- 



action among the organs of the vertebrate embryo 231 



c. Developmental rate and post-natal changes 245 



9. Continuity of the series from monstra in defectu through the single 



normal individual to monstra in excessu and finally identical twins. 247 



10. The necessity of a controlled or regulated environment in which to 



develop highly complex individuals 248 



11. Growth competition between the two components in double individuals 



and the time of occurrence of teratoma in man 251 



12. Cancerous growths and the general cessation of all normal growth in the 



old individuals 252 



13. General summary 255 



14. Literature cited 263 



1. INTRODUCTION 



In the present contribution an endeavor will be made to ana- 

 lyze the causes and conditions which determine the usual type of 

 structural expression or form. The ordinary progress of embry- 

 onic development gives rise to individuals of rather uniform struc- 

 ture, yet there may be numerous slight variations and defects in 

 the structural composition of various organs and parts. Minor 

 defects in structure are found in almost every individual of a group, 

 but rarely do two individuals present exactly the same kind or 

 degree of defects. These facts are readily recognized in a group 

 of human beings where small differences are easily appreciated, 



