Harris Hawthorne Wilder 425 



common knowledge that in animals (and plants) division may occur 

 in such a wa}^ that two or more bodies may be formed from what is 

 ostensibly one fertilized ovum. But hy a similar division, imperfectly 

 effected, the resulting bodies, instead of being complete twins or triplets, 

 may remain united together, frequently having a greater or less extent 

 of body in common." The italics are my own, and point out with 

 extreme clearness my views on the subject as explained above. Bateson, 

 however, continuing farther in his ideas, and believing that the com- 

 posite body must show a complete bilateral symmetry, assumes, as Fisher 

 did, the need of a situs viscerum inversum in one of the components, a 

 view which appears to be upheld in a few instances " but fails in others. 

 On this point he finally concludes that the amount of transposition 

 depends upon the time of separation, a view the precise meaning of 

 which is not clear in connection with his former explicit statement that 

 such cases arise from the division of " what is ostensibly one fertilized 

 ovum," and suggests an unwillingness to al^ide by his theory so clearly 

 expressed above. 



More recently this theory has been advocated by Sobotta, oo," although 

 he is uncertain at what point in early development the separation takes 

 place. iKs opposed to the belief that this point is that of the two-celled 

 stage, he suggests (1) that we do not know whether or not these blasto- 

 meres in the human species are " aquipotent; " (2) that it is difficult to 

 conceive of a force capable of separating these blastomeres that lie in 

 one egg, protected by the zona pellucida, the oviduct, etc.; (3) that if 

 one supposes a total separation, as in " eineiigen Zwillingen," it is hard 

 to understand how the chorion came to be single, as seems always to be the 

 case in such instances. He does not seem to consider these objections fatal, 

 however, and although he inclines to the belief that the separation takes 

 place during the later cleavage or in the " Keimblasenstadium," he 

 admits that " die Ursache der Doppelbildungen auf einem friiheren 

 Stadium zu suchen ist als im ersten Augenblick, wo sie der directen 

 Beobachtung zuganglich [sind] .'' On the other hand he says : " Vielleicht 

 kann audi bei stark verwachsenen Doppelbildung die Ursache der Storung 

 noch viel spater einwirken auf die Area embryonalis selbst und selbst 



" For this Bateson quotes Eichwald who finds that in thoracopagi " in al- 

 most every .case one of the bodies shewed some transposition of viscera, 

 though to a varying extent." (Pet. Med. Zeitsch., 1870, No. 2.) 



" The original paper of Sobottta has unfortunately not been accessible to 

 me, and I have relied entirely upon the review of the same given by Broman, 

 02. I hope that, in spite of this disadvantage, his views have been fairly 

 represented. 



