312 



Charles Zelen 



i 



Fig. 13 (x 216). 



Cleavage and Larval Stages from Isolated Blastonieres of the Tivo-Ccll Stage. 



A, nine-cell stage (rrrcup sbaped type); view from concave side. The cut 

 was made at one side of cleavage plane so that the fragment included one 

 blastomere plus part of the other. B, larva (age=z:33^ hours). Note single 

 lappet, solid enteron and absence of apical organ. C, larva (age=:33 hours). 

 Note solid enteron and absence of apical organ. D, larva (age^33^ hours). 

 E, larva (age=:r33 hours). F, larva (agez=33^ hours). G, larva (age=:33 

 hjours). The blastomeres were not completely separated and may have fused. 

 H, larva (age=47 hours). Note apical organ and small enteron. 



In three cases the embryos resemble the five just mentioned, ex- 

 cept that the archenteric mass is not as large and a slight blasto- 

 coele, crescentic in vertical section, is present (Figs. 13E, F) . 

 This blastocoele contains rounded and irregular mesenchyme cells. 

 There is no apical organ. The larvae do not differ widely from 

 the normal larva of about 24 hours though their age is 33 hours. 



