188 PRESSURE EFFECTS ON CLEAVAGE 



OBSERVATIONS AND TABULATION OF DATA: 



The observations here are purely qualitative and a series of sketches or photographs 

 should be made of several eggs whose cleavage planes have been altered. Then the eggs, 

 properly identified with their sketches, should be isolated in #2 Stenders and allowed to 

 develop as far as they will normally. Any variations from the controls should be indi- 

 cated by parallel sketches of experimentals and controls. 



DISCUSSION: 



Pfluger (1884) was probably 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 (1893) and O. Hertwig (1893) 

 repeated and refined the work on the frog's egg. None of these investigators left series- 

 sketches to indicate whether the grey crescent was involved, and the effect on subsequent 

 cleavages. Their concern was primarily with whether succeeding development would be 

 normal. 



The first cleavage in the frog's egg occurs about Z\ hours after insemination, the second 

 about 1 hour later and the third about \ hour after the second. There is an acceleration 

 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 4-cell stage, the cleavage 

 plane will be shifted. If the third cleavage, under pressure, is shifted to the vertical, 

 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 main- 

 tained 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. 



REFERENCES 



BORN, C. , 1894 - "Ueber neue Compressionversuche an Froscheiern. " Jahresbericht der Schlesischen Cesellschoft fur waterlan- 



dische Cultu. Zool. Bot. 

 DRIESCH, H. , 1892 - "Zur Verlagerung der Blostomeren des Echinodeneies. " Anat. Anz. 8:348. 



HERTWIG, O. , 1893 - "Ueber den Werth der ersten Furchungszellen fur die Organbildung des Embryo. " Arch. Mikr. Anat. 42. 

 MARSLAND, D. A . , 1956 - "Protoplasmic contractility in relation to gel structure. " Int. Rev. Cytology. 5:199. 

 MORGAN, T. H., 1910 - "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. , 1884 - "Ueber die Einwirkung der Schwerkraft und anderer Bedingungcn auf die Richtung der Zelltheilung. " Arch. 



Ges. Physiol. 34. 

 ROUX, W., 1895 - "Cesammelte Abhandlungen uber EntwicHungsmechanik der Organisman. " Leipzig. 



'Treasure your exceptions" 

 Bateson 



