28 BIOLOGICAL LECTURES. 



the posterior arm the posterior cell plate, the anterior rosette 

 series gives rise (at least in part) to the cerebral ganglia, the 

 shell gland and growing point come from the posterior member 

 of the second quartette (2d), the paired mesoblast bands and 

 the distal end of the intestine from the posterior member of 

 the fourth quartette (4d), the roof of the archenteron from the 

 remains of the four primary macromeres, its sides and floor 

 from the fifth and fourth quartettes respectively; in fact, so 

 many cells may be traced through to definite organs or parts 

 that one is justified in concluding that under normal conditions 

 every one of the earlier blastomeres gives rise to a particular 

 part. The constancy ^vith which differentiated cells give rise 

 to differentiated layers, regions, and organs is the most funda- 

 mental fact of determinate cleavage. 



What is the cause of determinate cleavage ? 



Such widespread, precise, and constant phenomena cannot, 

 of course, be due to chance; nor are they the result of uni- 

 versally acting mechanical causes, such as gravity or surface 

 tension. Certain indeterminate features of cleavage may be 

 directly referred to extrinsic factors or mechanical conditions; 

 e.g., the rotation of cells into the furrows between blastomeres 

 is probably referable to the principle of surface tension or mu- 

 tual pressure, the contour of cells is frequently the result of 

 intercellular pressure, the alternation of successive cleavages is 

 an expression of the principle of rectangular intersection of 

 cleavage planes, and this in turn may be due to the fact that 

 the nuclear spindle usually lies in the direction of the greatest 

 mass of protoplasm, and hence in the direction of least resist- 

 ance. These features, however, are neither constant nor differ- 

 ential. So far as the principle of surface tension is concerned 

 cells might rotate to the right or to the left indiscriminately, 

 yet in determinate cleavage the direction of rotation is per- 

 fectly constant. So, also, it frequently happens that successive 

 cleavages do not alternate in direction, and in such cases the 

 nuclear spindles often appear to lie in the direction of greatest 

 pressure. In general, the direction of teloblastic and non- 

 alternating cleavages can be referred only to peculiarities in 

 the protoplasmic structure of the cells, and, as I have pointed 



