THE PRODUCTION OF GRAI-TKD EMBRYOS. 99 



(b) Twins with a common blastoca-lr. 



(c) Twins with a reciprocal influence on their growth. 



(d) Fusion with partially double archentera. 



(e) True fusion with a single set of organs. 



(/) Single body with a second parasitic archenteron. 



Since little attention has been given to the early development 

 of such agglutinations and fusions, the text emphasizes the earlier 

 stages summarized as follows: 



Clusters of 2 to 20 eggs and blastomeres were successfully 

 agglutinated. The large clusters nearly always disappeared 

 either by the separation of the outer members or by the death 

 and disintegration of the inner ones. Small clusters of 2, 3 or 4 

 eggs or blastomeres survived and either remained agglutinated 

 or were fused. 



The eggs were agglutinated either at the egg stage or during 

 the formation of the blastula. In clusters- which remained 

 agglutinated and did not fuse, the members developed inde- 

 pendently, i. e., the polarity was not affected by the proximity 

 of the other, and the rate of development was not necessarily 

 different in the individual members of a cluster. 



Fusion occurred infrequently at the egg stage, but more 

 commonly took place during the blastula or later stages. Such 

 fusion involved either the body wall, one or more of the internal 

 organs or all of these. 



Fusion was frequently determined by the degree of compression 

 of the component eggs or blastulse, as well as the position and 

 angle of attachment. 



Embryos fused end to end frequently developed a single 

 archenteron, about twice the normal size, either with or without 

 blastopores. 



Embryos fused with their axes 90 degrees apart, frequently 

 fused in such a manner that their archentera united at the point 

 of contact. 



Embryos fused with their axes parallel and close together, 

 frequently fused in such a manner that the two archentera fused 

 into one, along their whole length. 



The study of individual clusters served to show that these 

 types of agglutination and fusion represented regulative and 



