98 A. J. GOLDFARB. 



Two eggs fused into one pluteus with independent archentera, 

 one very much larger than the other. 



Two eggs fused completely into one pluteus body with one 

 set of organs. 



Other clusters showed distinct retrogressive and involutional 

 changes of which the following are a few examples : 



Four blastulae were agglutinated in a row. The end ones 

 enlarged, the inner ones became small. One of these small 

 blastulee developed an archenteron which later shrank and 

 disappeared completely. Certain other changes occurred which 

 altered the character of the group so that ultimately three minute 

 blastulae were crowded together at one point on the periphery 

 of a large blastula. 



Two large fused blastulae were attached to a third blastula 

 very much smaller. The three fused into one body with three 

 independent archentera. The next day the small archenteron 

 disappeared, skeletal rods differentiated on one side of the body 

 constituting a fusion of a pluteus and a gastrula. Two days 

 later the parts had fused more completely, into a single body 

 with a single normal-sized archenteron and a single skeleton. 



Two eggs developed into fused gastrulae and later into fused 

 plutei, attached by their oral surfaces. One of the plutei 

 decreased in size while the other increased correspondingly, the 

 total linear dimensions remaining constant. The smaller pluteus 

 ultimately became less than one quarter its original size. 



SUMMARY. 



Prior to this work, no one had succeeded in fusing at will the 

 eggs of any animal found on this side of the Atlantic. By a 

 modification of the Herbst-Driesch method, described in the 

 text, it was possible to agglutinate and to fuse relatively large 

 numbers, namely 10 to 40 per cent, of the sea urchin Arbacia 

 punctulata. 



Such clusters whether studied in mass cultures or in isolated 

 groups, developed into all the types of larvae described by 

 Driesch for Echinus microtuberculatus and Sphcerechinus granu- 

 laris, namely: 



(a) True twins. 



