192 PRESSURE EFFECTS ON CLEAVAGE 



DISCUSSION ; 



Pfluger (188^) was protably the first to discover that vertically applied pressure 

 will alter the normally horizontal third cleavage plans of the frog's egg and will make it 

 vertical, as are the first two cleavages. This means that in the 8-cell stage, all eight 

 cells extend from the animal to the vegetal pole. Driesch (1892) applied pressure to the 

 cleaving sea-urchin egg in an attempt to alter its cleavage plane. Born (1895) and 0. 

 Hertwig ( l895 ) repeated and refined the work on the frog's egg. None of these investiga- 

 tors left series-sketches to indicate whether the grey crescent was involved, and the ef- 

 fect on suhsequent cleavages. Their concern was primarily with whether succeeding develop- 

 ment would be normal. 



The first cleavage in the frog's egg occurs about 2^ hours after insemination, the 

 second about 1 hour later and the third about ^ hour after the second. There is an ac- 

 celeration of cleavages. Each of the cleavages is at right angles to the preceding 

 cleavage, and the spindle axis lies in the longest protoplasmic axis (see the laws of 

 Hertwig, Sachs, and Balfour in the Glossary). The third cleavage is normally horizontal 

 (equatorial), and the fourth is again meridional (vertical). 



If the protoplasmic axis of the blastomeres is shifted at the i^-cell stage, the cleav- 

 age plane will be shifted. If the third cleavage, under pressure, is shifted to the verti- 

 cal, and the egg is then released from pressure, the next cleavage (normally vertical) will 

 tend to be horizontal. 



Generally eggs which have been subjected to this type of unequal external pressure 

 will survive perfectly well and will develop quite normally providing the pressure is not 

 maintained too long and it does not rupture the surface coating of the egg. Such a shift 

 in cleavage pattern as generally occurs means a shift in the distribution of nuclei. 

 Since abnormal embryos are not generally produced by altering one of the cleavage planes, 

 it must be assumed that there is no qualitative distribution of the nuclear material in 

 these early blastomeres. 



BEFERENCES : 



Born, G., l89^ - "Ueber neue Compressionversuche an Froscheiern. " Jahreabericht der 



Schlesischen Gesellschaft fur waterlandische Cultu. Zool. Bot. 

 Driesch, H. , I892 - "Zur Verlagerung der Blastomeren des Echinodeneies. " Anat. Anz. StJ*^. 

 Hertwig, 0., 1895 - "Ueber den Werth der ersten Furchungszellen fur die Organbildung des 



Embryo." Arch. Mikr. Anat. 42. 

 Morgan, T. H. , I9IO - "The effects of altering the position of the cleavage planes in 



eggs with precocious specification." Arch. f. Ent. mech. 29. (See also "Experimental 



Embryology" 1927.) 

 Pfluger, E., I88U - "Ueber die Einwirkung der Schwerkraft und anderer Bedingungen auf die 



Rlchtung der Zelltheilung. " Arch. Ges. Physiol. 5U, 

 Roux, W., 1895 - "Gesammelte Abhandlungen uber Entwicklungamechanik der Organisman." 



Leipzig. 



