2/8 CELL-DIVISION AND DEVELOPMENT 



mechanical means. The history of division in the cambium-cells 

 and columnar epithelium seems to show that neither direct pressure 

 nor the shape of the cells caused by it can be the ultimate cause. 

 The succession of divisions, always in the same plane, in apical cells 

 and in teloblasts, is directly related with a deeply lying law of growth 

 that affects the whole developing organism, and we cannot at pres- 

 ent distinguish in such cases between cause and effect ; for whether 

 the apical growth of the body as a whole is caused by local condi- 

 tions within the apical cells, or the reverse, is undetermined. This 

 unsatisfactory result shows how far we still are from an understand- 

 ing of the fundamental laws of growth and their relation to cell- 

 division, and how vast a field for experimental research lies open in 

 this direction. 



B. Promorphological Relations of Cleavage 



The cleavage of the ovum has thus far been considered merely as 

 a problem of cell-division. We have now to regard it in a far more 

 interesting and suggestive aspect ; namely, in its morphological rela- 

 tions to the body to which it gives rise. From what has been said 

 thus far it might be supposed that the ^^^ simply splits up into indif- 

 ferent cells which, to use the phrase of Pfluger, have no more definite 

 relation to the structure of the adult body than have snow-flakes to the 

 avalanche to which they contribute. Such a conclusion would be 

 totally erroneous. It is a remarkable fact that in a very large num- 

 ber of cases a precise relation exists between the cleavage-products 

 and the adult parts to which they give rise ; and this relation may 

 often be traced back to the beginning of development, so that from 

 the first division onwards we are able to predict the exact future of 

 every individual cell. In this regard the cleavage of the ovum often 

 goes forward with a wonderful clock-like precision, giving the impres- 

 sion of a strictly ordered series in which every division plays a defi- 

 nite role and has a fixed relation to all that precedes and follows it. 



But more than this, the apparent predetermination of the embryo 

 may often be traced still further back to the regions of the undivided 

 and even unfertilized ovum. The egg, therefore, may exhibit a dis- 

 tinct promorphology ; and the morphological aspect of cleavage 

 must be considered in relation to the promorphology of the ovum 

 of which it is an expression. 



I. Promorphology of the Ovum 



{a) Polarity and the Egg-axis. — It was long ago recognized by 

 von Baer ('34) that the unsegmented egg of the frog has a definite 

 egg-axis connecting two differentiated poles, and that the position of 



