CH. XI] EFFECT OF INJURING A BLASTOMERE 



injured, the surface may be subsequently covered by ectoderm 

 that grows directly from the developing half over the injured 

 portions. " Postgeneration" now begins in the cellulated in- 

 jured half and ultimately the missing half of the embryo is 

 formed. The surface ectoderm is first postgenerated either by 

 direct overgrowth from the uninjured to the injured side, or 

 by the formation of ectoderm from the cells of the newly cellu- 

 lated yolk. The missing half of the medullary folds appears 

 very quickly. Half a day or a night is often sufficient to change 

 a hemiembryo lateralis into a whole embryo with a complete 

 medullary plate. The mesoblast grows over to the injured 

 half, but increases in length and breadth by the addition of 

 new cells from the cellulated yolk. The formation of new 

 mesoderm takes place only along the free edge of that already 

 formed. The growth is in a dorso-ventral direction. 



The archenteron is postgenerated in a way very different 

 from the way in which the archenteron of the normal embryo 

 is formed. The lacking half of the archenteron arises in 

 connection with the half of the archenteron already present 

 in the hemiembryo. The yolk-cells of the injured half be- 

 come radially arranged and a slit appears in the postgenerated 

 half extending out from the archenteron of the hemiembryo. 

 The cells surrounding the slit arrange themselves into a lining 

 layer and the slit opens to form the missing half of the archen- 

 teron. In general we may say that in the postgeneration 

 of the organs of the injured half, the changes always proceed 

 from the already differentiated germ-layers of the hemiembryo, 

 and the postgeneration takes place where the exposed surfaces 

 of the germ -layers touch the newly cellulated yolk-mass of the 

 injured half. 



FURTHER EXPERIMENTS 



(By Hertwig, Endres and Walter, Schultze, Wetzel, Morgan) 



We may next consider the work of others, who have, after 

 Roux, repeated the same experiment and made further varia- 

 tions of it. Lastly, before a final conclusion can be reached as 

 to the interpretation of the results, we must carefully examine 

 the evidence from similar experiments on other forms. We 



