i88 CYTOLOGY chap. 



formation of the various tissues and structures of the body ? A discussion 

 of this question would again lead us far beyond the scope of this book, 

 and would indeed involve nearly the whole subject of experimental 

 embryology. Here we can only allude to the question of the so-called 

 " organ-forming substances," referred to on p. 163. Lack of space and 

 the uselessness of repeating what has already been made the subject 

 of recent text-books must restrict us to a very brief, and therefore 

 necessarily dogmatic, summary. The reader is referred for further 

 information to Korschelt and Heider's Lehrbuch (1902), or to the smaller 

 text-book of Jenkinson (1909), where all the essential facts are given, and 

 where references to the more important original works may be found. 



It is found that a single blastomere isolated from an embryo of the 

 i6-cell stage (termed a tV blastomere) of a sea-urchin will hve and develop, 

 at least as far as gastrulation, as if it were a whole miniature egg. It 

 is only blastomeres from the lower or vegetative pole of the cleaving 

 egg, however {i.e. that part of the egg from which gastrulation starts in 

 a normal, whole embryo), which will gastrulate. Cells from other parts 

 develop irregularly and do not gastrulate; | blastomeres, however, 

 whichever part of the egg they are taken from, will develop into embryos 

 which gastrulate. If the egg is divided into its blastomeres at the two- 

 cell stage, both develop into a perfect normal, though dwarf, pluteus. 



This is expressed by saying that the blastomeres of an Echinoid egg 

 are equipotential as far as the eight-cell stage, then gradually become 

 inequipotential. The eggs of a large number of animals can be arranged 

 in a series, according as to how long they retain their totipotentiaUty, 

 down to the forms where even the undivided egg is inequipotential in 

 its various parts. 



Thus in the radially symmetrical Ctenophora, if a segment of the 

 cytoplasm of the undivided egg is removed, the resulting larva lacks the 

 organs on the radius represented by the removed egg cytoplasm. 



The eggs of several Molluscs and Chaetopods exhibit a swelling of 

 the cytoplasm (after fertihzation but before cleavage has begun) termed 

 the yolk lobe. If this is cut off, the egg will nevertheless develop to a 

 certain stage, but the resulting larva, though it may become free- 

 swimming, does not develop any mesoderm. 



These and many other experiments have led to the hypothesis of 

 the presence of " organ-forming substances " in the egg cytoplasm exhibit- 

 ing a stratified arrangement, generally according to their specific gravities. 

 For instance, ectoderm-forming substance is most concentrated towards 

 the upper pole of the egg and least concentrated towards the lower 

 pole, and endoderm-forming substance has the reverse arrangement. 

 In the cleaving Echinoderm egg, up to and including the eight-cell stage, 

 there is sufficient of all organ-forming substances in all the blastomeres 



