412 TRANSACTIONS OP SECTION D. 



apparently quite undifferentiated in the earlier periods of development. Now, 

 in our discussion of Driescli's experiments we have seen that when development 

 reaches a certain point, the embryo ceases to be equipotential in all its parts. 

 In the case of the sea-urchin this point is reached when the primary mesenchyme 

 cells are being formed ; now Schaxel has shown that the nuclei of these cells 

 are surrounded by the familiar blush of chromatin, which points to the con- 

 clusion that the nuclei are again emitting organ-forming materials into the 

 cytoplasm. It is after this event that we find that the upper half of the 

 blastula is incapable of forming a gut. We cannot, however, conclude that 

 ectodermic and endodermic substances are first formed at tliis stage, because 

 then we could not account for the fact that in an earlier period of develop- 

 ment any part of the blastula will, if cut off, heal up and form a small 

 blastula cajDable of forming a gut. Rather the evidence forces us to assume 

 that ectodermic and endodermic organ-forming substances begin to be formed 

 shortly after fertilisation and continue to be formed for some time, but that at 

 first they are not separated from one another, so that when segmentation 

 occurs they exist side by side in the same cell ; as development proceeds, the 

 endodermic substances become gradually segregated towards one pole and the 

 ectodermic substances towards another. We must think of the cell-walle as 

 permeable to these substances ; indeed, we must regard the protoplasm of the 

 embryo as a whole in spite of its apparent division into cells. The best proof 

 of this view is furnished by Herbst's well-known experiment of exposing the 

 developing eggs of the sea-urchin to the action of the salts of lithium. We all 

 know that eggs which have undergone this treatment develop into motionless 

 blastulse, whose walls later become differentiated into two regions — one corre- 

 sponding to the ectoderm and one to the endoderm of a normal embryo. Such 

 embryos, if replaced in normal sea-water, acquire the power of motion, and the 

 part corresponding to the gut of a normal gastrula often shows signs of differ- 

 entiation into oesophagus, stomach, and intestine — turned inside out. This 

 'lithium larva,' however, is not formed unless the eggs are placed in the 

 lithium solution immediately after fertilisation, or at least during the early stages 

 of segmentation, and continue in it until they attain the blastula stage. Now, as 

 the intensity of the action of the lithium salts increases, so does the proportion 

 of the wall of the lithiimi blastula, which takes on endodermic characters till in 

 extreme cases only a minute button representing the ectoderm remains, and in 

 a few cases even this can disappear. It is obvious that the effect of the lithium 

 is to increase the amount of endoderm-forming substance, and therefore this 

 substance must be manufactured during the period of the egg's exposure to 

 lithium salts; that is, after fertilisation up till the formation of the blastula. 

 We see then that in eggs of this type the emission of organ-forming substances 

 goes on after fertilisation : that these are only gradually localised and, pari 

 passu, with their restriction to definite regions the power of all parts of the 

 embryo to develop the whole organism is lost. Even Driesch was able to show 

 that when 16-cell .segmentation stages are broken into groups of cells, though 

 all groups of any size can form miniature larvae, those groups which belong to 

 the lower half of the blastula develop more easily than the others, since their 

 cells contain a larger proj^ortion of endodermic substance. 



The discovery that, in the case of some animals at leafit, the emission of 

 organ-forming substances from the nucleus goes on after fertilisation encourages 

 the thought that even in those cases where the organ-forming substances appear 

 all to be fonned before fertilisation and the nuclei are relatively passive during 

 early development the nuclei may later resume their active role. Now, in two 

 cases where, by the separation of the first two blastomeres, we are enabled to 

 get half-embryos, it can be shown that the missing half is later regenerated. 

 This is true of the frog, and is also true of the cten.ophore. The ctenophore 

 furnished us with such a beautiful instance of the limited potentialities of 

 isolated blaetomeres that it comes as a shock to learn that the exquisite half- 

 embryos produced by separating the first two blastomeres can regenerate the 

 missing half. This fact was first noticed by Chun, but has been confirmed by 

 Mortensen. Now, the most natural way to explain this regenerative power is to 

 attribute it to a renewed activity on the part of the nuclei in producing organ- 

 forming substances. If we accept this view a good many curious facts about 



