192 DYNAMICS OF LIVING MATTER 



that the sohd constituents of the body, e.g. the skeleton, are formed 

 later on, and that its form can, of course, be only indirectly pre- 

 determined in the egg. 



We are therefore forced to conclude that for the transmission of 

 the hereditary qualities no complicated or morphological structure is 

 required in the sexual cells. This harmonizes with the idea already 

 gathered from the preceding parts of this lecture, that chemical 

 conditions are the bearers of hereditary qualities in the egg, for the 

 instincts as well as the form of the body. 



Driesch has shown that when a single cell of the two- or four-cell 

 stage in the development of the egg in the sea urchin is isolated, this 

 cell not only develops into a pluteus, but the mode of development 

 is not essentially different from that of the intact egg.* It would be 

 merely a play on words to speak in such a case of regeneration. The 

 development in the early stages consists in successive divisions of each 

 cell and the creeping of each of these cells to the surface, so that finally 

 the mass of cells thus formed is a sphere with cells at the surface, while 

 a space in the center remains free from cells and is filled with a liquid. 

 The cells at the surface of this hollow sphere then form ciha at their 

 external surface, and in this stage the larva, which now begins to swim 

 through the motion of the cilia, is called a blastula. The next stage 

 in the development is the growing in at one spot of the blastula, of a 

 group of cells, into the hollow space of the sphere; and the cells thus 

 growing in, form finally an inner lining of the cells of the blastula. This 

 process is called the gastrulation, inasmuch as this inner Uning is the 

 beginning of the alimentary tract of the larva. At this stage the larva 

 is called the gastrula. Later on large cells are formed, the mesenchyme 

 cells, which creep to certain places in the gastrula, giving rise to the 

 skeleton, probably through the secretion of CaCOj, or of a substance 

 that leads to the formation of CaCOj. According to Driesch, the pro- 

 cess of development of a pluteus from an isolated cell of a two- or four- 

 cell stage of the sea urchin's egg occurs in practically the same way, 

 as in the case of the development of an intact egg; except that the 

 larva developing from a single cell of the two- or four-cell stage is smaller 

 than the normal larva, having only one half or one fourth the mass 

 of the latter. There may be also slight differences in the development, 

 owing, as I believe, to a kind of hysteresis, inasmuch as the side of the 

 cell which was in contact with the other cells of the egg before the blas- 

 tomeres were separated, acts possibly a little differently from the other 

 sides. These experiments of Driesch are of great importance, inas- 



* For the extensive literature on this subject, see E. B. Wilson's book, The Cell, New 

 York, 1900 ; or T. H. Morgan's book on Regeneration, New York, 1900. 



