go THE BIOLOGY OF TWINS 



a single blastomere of the four-celled stage" as "spon- 

 taneous blastotomy." The very meaning of this term 

 involves the idea of physical separation of blastomeres 

 and is therefore quite inappropriate in connection with 

 the idea that had been expressed by the joint authors. 

 I quite agree that no true "blastotomy" occurs, but 

 I would maintain that there is much evidence for and 

 little against the idea that the cleavage process of the 

 armadillo is determinate, that the cell descendants of 

 each blastomere of the four-cell stage constitute 

 essentially a quadrant of the vesicle (including tropho- 

 blast, ectoderm, endoderm, etc.), and that therefore the 

 inherited characters of each embryo are dependent upon 

 the particular quadrant, or parts of different quadrants, 

 from which it is derived. 



Patterson, however, totally abandons the idea, 

 formerly entertained at least tacitly by him, that there 

 is any connection between the four embryos and the 

 four blastomeres. This change of opinion is doubtless 

 due to the observation that budding appears to occur 

 in accordance with the bilaterality of the uterus rather 

 than in accordance with any lines of demarkation 

 predetermined in the egg. 



In a recent paper Wilder (1916), in discussing the 

 armadillo results, continues to maintain the view that 

 there is a real connection between the four blastomeres 

 and the four fetuses. He says that it is now clearly 

 evident that all ideas of physical separation of these 

 blastomeres, a definite blastotomy, does not take place, 

 yet many things still point to the conclusion that a 

 similar condition is obtained through some form of 

 differentiation, and that each of the separate embryonal 



