186 



INTERNAL FACTORS 



IV. i 



right angles to one another and to the coverglass, but whether 

 meridional or not does not appear. The third furrows were again 

 at right angles to the coverglass and at 45 to the first two; a flat 

 plate of eight cells was thus formed. The next division resulted in 

 the formation of sixteen equal cells still all lying in the same plane ; 

 the formation of micromeres is thus suppressed. Such eggs gave 

 rise to normal larvae. 



In the case just quoted the egg-membrane was intact; but similar 

 results were obtained when it was broken. The second furrow was 

 sometimes at right angles to, sometimes parallel to, the first, the 

 third at right angles to both in the latter case, and the fourth 



d c f 



FIG. 94. The effect of heat upon the segmentation of the Echinoid 

 egg. a, b, c, d, four successive stages in the segmentation of the same egg 

 of Echinus ; e, /, two successive stages in the division of the same egg of 

 Sphaerechinus. (After Driesch, 1893.) 



parallel to the third ; but if the pressure was released the blasto- 

 meres became rounded off and the sixteen-celled stage consisted 

 of two plates of eight cells each ; one or two micromeres were 

 sometimes formed. The development of these eggs is also quite 

 normal. These results have been confirmed by Ziegler. 



In this, as in the similar experiment of Hertwig on the Frog's 

 egg, the normal distribution and arrangement of the nuclei is 

 interfered with without prejudice to the normality of subsequent 

 development. Nuclear division cannot therefore, Driesch contends, 

 be a qualitative process. Boveri has also shown in similar 

 fashion that the order of segmentation may be disarranged and 



