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H. H. NEWMAN 



in which the primary archenteron has evidently undergone very 

 early fission, and there is, in addition, a small secondary archen- 

 teron at the apical end. There have been seen in partheno- 

 genetic cultures not a few cases in which paired archentera arise 

 in such a way as to produce symmetrical conjoined twins. In 



19 



22 



24 



Fig. 19 A typical gastrula about forty-eight hours old. 



Fig. 20 A very abnormal twin gastrula with paired primary archenteron and 

 a small secondary archenteron at the apical pole. 



Fig. 21 Gastrula with normal primary archenteron and three small second- 

 ary archentera near the apical end. 



Fig. 22 Very irregular gastrula with three grades of archentera. 



Fig. 23 A type of larva with an 'apical plate' which is merely a secondary 

 basal region in which gastrulation has not yet taken place. 



Fig. 24 A type of larva doubtless resulting from a condition like that shown 

 in figure 23, in which a minimal secondary gastrulation has occurred. 



these cases it seems evident that the formation of a primary 

 archenteron, unlike the conditions previously described, has been 

 inhibited and secondary archentera arise at equal distances from 

 the site of the original basal end of the blastula. An interesting 

 series of these mirror-image conjoined twins is shown in figures 

 25 to 28. Figure 25 shows a larva with two archentera both 



