326 



EMBRYOLOGY 



additional small cells, which are added to the four ectoderm cells already 

 present, does not agree with the processes in Clepsine. The " neurone- 

 phroblast" and the "mesoblast" each divide into two cells, which are 

 placed symmetrically in respect to the middle line. The edges of the 

 macromeres arch up more or less over the small blastomeres, so that 

 these at times appear to be embedded in them, a process that also takes 

 place in like manner in Clepsine. The fate of the different blastomeres 



Fig. 151,.-^ anil B, cleavage stage and an embryo at the time of hatching of 

 Hephelit vu^aris (after BOtschli). cct, ectoderm; cnt, entoderm; Ifsfr, germ 

 bands; nm/ir, macromeres ; mifcr, micromeres; m, mouth-opening; t, pharynx. 



could not be followed farther than this, though it is to be assumed 

 that the further differentiation is the same as in Clepsine. At all events, 

 two " germ bands " are also formed here (Fig. 154), which extend from 

 behind forwards and there (m the region of the future mouth) unite. 

 The metamorphosis of the entoderm is important, and in determining 

 the entire shape of the animal, significant. To the entoderm cells first 



