Localization of the N emertine Egg. 



319 



Fig. 18 (x 216). 



Larvce from Portions of the Egg oi tin Sixtecn-Ccll Stage. 



A, larva (agc:r=46j^ hours) from the upper four cells (one injured). 

 Note presence of apical organ and absence of enteron. B, larva (age^46y2 

 hours) from the lower twelve cells of the same egg. Note that egg is a solid 

 mass with division into ectodermal and endodermal cells. C, larva (age 46^4 

 hours) from upper four cells. Note presence of apical organ and absence of 

 enteron. D, larva from lower twelve cells of the same egg; same age. Note 

 solid enteron and absence of apical organ. 



tilization an elongated very actively swimming larva with a long 

 apical cilium had developed. The embryo showed after stain- 

 ing and mounting a well developed apical plate. The anterior 

 end of the body is occupied by a blastocoele containing a few scat- 

 tered free cells. The posterior end is a dense mass of cells, with 

 no signs of an ingrowth of these to form an archenteron (Fig. 

 18A). From the lower tweh^e cells at the same time there was 

 developed a ciliated elongated embryo, with only a slight rotation, 

 and no forward movement of the body. The embryo is a solid 

 mass of cells, the only differentiation visible being a difference be- 

 tween the cells at the two ends. Those near one end have the 

 typical histological endoderm characters of the normal larva, 

 while those near the other end have ectoderm characters (Fig. 

 18B). 



The characters of the two larvae in this case again show the 

 presence of the apical-basal differentiation described for the last 



