CELL-DIVISION AND DEVELOPMENT. 133 



This, then, is what is meant by teloblastic growth, and it 

 will have been noticed from the description that the division of 

 the teloblasts is not a division into equal parts, and that the suc- 

 cessive division planes are not at right angles but parallel to 

 one another. The arrangement of the cells may, however, be 

 reduced to an accordance with the principle of orthogonal tra- 

 jectories, the metanaupliar region being regarded as a quad- 

 rangular superficies, in which case the trajectories would be 

 straight lines cutting one another at right angles, and it may 

 be presumed that the exception to the third factor of Sachs' 

 law is only apparent. In fact, if this factor be defined as a 

 tendency for the cells to divide so as to be arranged in 

 orthogonal trajectories, the exception no longer exists, there 

 being, then, no necessity for successive rectangular divisions. 

 The divisions by which the row of teloblasts is formed origi- 

 nally may be regarded as one set of trajectories which are 

 formed once and for all, the other set of trajectories being 

 produced by successive divisions. 



There is still left, however, the exception to equal divisions, 

 and in addition it may be pointed out that teloblastic division 

 forms an exception to Hertwig's law since the successive spin- 

 dles form not at right angles, but parallel to the lines of pres- 

 sure, as it has been pointed out that the teloblasts are being 

 continually forced backward over the surface of the yolk and 

 are, therefore, subject to a pressure acting on them in an 

 antero-posterior direction. It does not, indeed, seem possible 

 to account for the peculiarities of teloblastic growth on any of 

 the mechanical hypotheses at present at our disposal. Sachs' 

 law, even if modified as suggested, and the law of minimal 

 surface areas, while explaining the arrangement of the cells, 

 do not explain why this arrangement should have been brought 

 about by teloblastic division, since it might have been accom- 

 plished by a succession of rectangular divisions. It seems in 

 the Crustacea to be a provision for the rapid growth of the 

 metanaupliar region of the body, and at present it must be 

 conceded that our knowledge of cell-mechanics is too super- 

 ficial to permit of an explanation of it on a purely mechanical 

 basis. 



