ELEMENTARY MORPHOGENESIS 39 



about fifty cells lose contact with their neighbours and 

 leave the surface of the globe, being driven into the interior 

 space of it. Not very much is known about the exact 

 manner in which these changes of cellular arrangement 

 are carried out, whether the cells are passively pressed by 

 their neighbours, or whether, perhaps, in a more active 

 manner, they change their surface conditions ; therefore, 

 as in most ontogenetic processes, the description had best 

 be made cautiously in fairly neutral or figurative words. 



The cells which in the above manner have entered the 

 interior of the blastula are to be the foundation of important 

 parts of the future organism ; they are to form its connective 

 tissue, many of its muscles, and the skeleton. " Mesenchyme," 

 i.e. " what has been infused into the other parts," is the 

 technical name usually applied to these cells. We now 

 have to learn their definite arrangement. At first they lie 

 as a sort of heap inside the cell wall of the blastula, inside 

 the "blastoderm," i.e. skin of the germ. But soon they 

 move from one another, to form a ring round the pole at 

 which they entered, and on this ring a process takes place 

 which has a very important bearing upon the whole type of 

 the organisation of the germ. You will have noticed that 

 hitherto the germ with regard to its symmetry has been a 

 monaxial or radial formation ; the cleavage stages and the 

 blastula with its mesenchyme were forms with two different 

 poles, lying at the ends of one single line, and round this 

 line everything was arranged concentrically. But now 

 what is called " bilateral symmetry ' is established ; the 

 mesenchyme ring assumes a structure which can be 

 symmetrically divided only by one plane, but divided in 

 such a way, that one-half of it is the mirror image of the 



