56 



CELL-DIVISION AND GROWTH 



II. i 



Driesch lias observed that in the egg of Echinus cell-division 

 may be wholly or partially suppressed by pressure, and also by 

 diluting- the sea-water. Nuclear division continues (Fig. 33). 



Morgan has found that the egg of another sea-urchin (Arlacia) 

 will not segment in a 2 % solution of salt in sea- water ; on 

 replacing the eggs in sea-water, however, the nucleus divides 

 with great rapidity several times, and this is followed by cell- 

 division. So Loeb notices that the eggs when treated in this 

 way, and brought back to their normal medium, divide simul- 



taneously into four. The egg 

 of the fish Ctenolahrus; (accord- 

 ing to the same author) behaves 

 in a similar fashion when first 

 deprived of, and then restored 

 to oxygen. 



TI QO v i,- c Graf, again, has seen the re- 



FIG. 33. Echinus : suppression of ' & . 



cell-division by pressure, b, and by union of sister cells and nuclei 

 uce8ion continues - in the eggs of Arlacia released 



from pressure. 



Three distinct agencies mechanical pressure, increase of os- 

 motic pressure, and decrease of osmotic pressure are all capable 

 of effecting this interesting change in the usual relations of cell 

 and nucleus. We can only guess at the real cause, and surmise 

 that it will be found in an alteration of internal and external 

 surface tensions. 



Q 

 (After Driesch, 1893.) 



LITERATURE. 



NOTE. For a complete bibliography of segmentation the well-known 

 textbooks of 0. Hertwig and Korschelt and Heider must be consulted. 

 The literature of ' spiral' segmentation is given by Robert (quoted below). 



F. M. BALFOUR. Comparative Embryology, London, 1885. 



G. BERTHOLD. Studien iiber Protoplasmamechanik. VII. Theilungs- 

 richtungen und Theilungsfolge, Leipzig, 1886. 



G. BORN. Ueber Druckversuche an Froscheiern, Anat. Am. viii, 1893. 



T. BOVERI. Die Entwickelung von Ascaris megalocepliala mit beson- 

 derer Riicksicht auf die Kernverhaltnisse, Festschr. Kupffer, Jena, 1899. 



E. G. CONKLIN. Protoplasmic movement as a factor of differentiation, 

 Woods Holl Biol. Lect., 1898. 



