22O REGENERATION 



nuclei, or from cells without walls that have emigrated from the 

 developing half. Around these nuclei, as centres, the protoplasm 

 (with its contained yolk) of the injured half breaks up into cells. This 

 cellulation of the yolk may take place in different eggs at different 

 times. In some cases it may not have appeared as late as the gas- 

 trula stage of the uninjured half; in others, it may take place at the 

 time when the uninjured half is segmenting. 1 The formation of the 

 cells in the injured half begins always near the developing half, and 

 extends thence into the injured parts. The new cells are of different 

 sizes, but are larger than those of the uninjured half. 



The cellulation of the yolk takes place only in the least injured 

 parts of the protoplasm. Where the protoplasm and yolk have been 

 much injured, they are changed over by the second method of reor- 

 ganization. This part of the blastomere is either actually devoured 

 by wandering cells, or is slowly changed under the influence of the 

 neighboring cells, so that it becomes a part of these cells. 



The surface of the injured half is covered over by ectoderm that 

 grows directly from the developing half (third method of reorganiza- 

 tion), at least this happens where the protoplasm has been much 

 injured. In other parts of the injured half the new cells that have 

 appeared in this part, and that lie at the surface, become new 

 ectoderm. 



Post-generation now begins in the reorganized and cellulated 

 half ; the cells become changed over into the different layers and 

 organs that make the new half-embryo. A few hours or a night is 

 sometimes sufficient to change a hemi-embryo into a whole embryo. 

 The new half-medullary fold develops from the new ectoderm to 

 supplement the half already present. The mesoblast appears over 

 the side. Its upper part seems to come from the uninjured meso- 

 derm that has grown over to the other side, but this is added to at the 

 free edge by cells that belong to the newly cellulated part. The new 

 differentiation is, in general, in a dorso-ventral direction. The lack- 

 ing half of the archenteron arises in connection with the half of the 

 archenteron already present in the hemi-embryo. The yolk cells 

 arrange themselves radially, and a split appears in the post-gen- 

 erated part, extending from the archenteron of the hemi-embryo. 

 The split opens, and the new half-archenteron appears. In general, 

 Roux states, the post-generation of the organs of the injured half 

 proceeds from the already differentiated germ-layers of the hemi- 

 embryo. The post-generation begins where the exposed surfaces of 

 the germ-layers of the hemi-embryo touch the newly cellulated regions 

 of the injured half. 



1 This difference is due, I suppose, to the amount of injury that the nucleus of the 

 injured half may have suffered. 



