Harris Hawthorne Wilder 463 



series differing geometricall}^ from one another and corresponding to 

 variations in the place of separation of the first two blastomeres, or in 

 their relative position. 



5. Unequal doiihle monsters (aidosite and parasite) are the result of 

 a secondary fusion of two embr3'os, owing to a too great contiguity. It 

 is probable that these are at first duplicate twins, the enclosure of which 

 within a common chorion would furnish the crowded conditions neces- 

 sary for such a fusion. 



B. On the Aechitectuke (Promorphology) of the Ovum. 



1. The normal mammalian egg, at least from the beginning of cleav- 

 age, possesses a definite architectural plan, having a fixed relation to 

 that of the adult body. It seems probable that this plan is bilateral, and 

 that, as in the frog's egg, the first two blastomeres are right and left, 

 and become the progenitors of the two sides of the adult, except that 

 here they must give rise also to the two halves of the chorion and other 

 extra embryonal parts. 



3. A change of relationship in the early blastomeres w^ill modify the 

 development of each, as has been shown experimentally in the case of 

 lower animals. In changes affecting the first two blastomeres, we may 

 draw the following conclusions: 



a. When they remain together in the normal position, the mutual con- 

 tact upon the inner sides causes each to develop as a bilateral half. 



b. When they separate and the inner sides, once in contact with each 

 other, become external, i. e., placed in the same relations as are the other 

 sides, each will develop the other half body as in the case of the entire 

 ovum, and produce duplicate twins. This shows that the power to 

 develop an entire organism when placed in the proper relations is re- 

 tained by each of the early blastomeres up to a certain point, as has been 

 proven experimentally to be the case in numerous lower animals. 



c. When a separation of the first two blastomeres is incompletely 

 effected, the parts that lose the contact relations develop independently 

 as parts of entire individuals, while the parts that remain in normal 

 contact develop each a half as usual. The various possibilities of partial 

 separation give rise to the various types of symmetrical double monsters 

 {diplopagi) . 



C. On the Composition of the Germ-Plasm, and the Size-Limit 

 OF Hereditary Control. 



1. It is undeniable that the facts presented in this paper point to some 

 form of preformation, that is, to a mechanism in the nucleus of the 



