III. i GRAVITATION 89 



M. MOSZCOWSKI. Zur Analysis der Schwerkraftswirkung auf die 

 Entwicklung des Froscheies, Arch. mikr. Anat. Ixi, 1903. 



E. PFLUGER. Ueber den Einfluss der Schwerkraft auf die Teilung 

 der Zellen, Pfluger's Arch, xxxi, xxxii, xxxiv, 1883. 



W. Roux. Ueber die Entwickluug des Froscheies bei Aufhebung der 

 ricbtenden Wirkung der Schwere, Breslait artz. Zeitschr., 1884 ; also 

 Ges. Abh. 19. 



W. Roux. Bemerkung zu 0. Schulze's Arbeit tiber die Nothwendig- 

 keit, etc., Arch. Ent. Mech. ix, 1900. 



W. Roux. Das Nichtno'thigsein der Schwerkraft fur die Entwicklung 

 des Froscheies, Arch. Ent. Mech. xiv, 1902. 



W. Roux. Ueber die Ursachen der Bestimmung der Hauptrichtungen 

 des Embryo irn Froschei, Anal. Anz. xxiii, 1903. 



0. SCHULZE. Ueber die unbedingte Abhangigkeit normaler tierischer 

 Gestaltung von der Wirkung der Schwerkraft, Verh. Anat. Ges. viii, 1894. 



0. SCHULZE. Ueber die Nothwendigkeit der freien Entwicklung des 

 Embryo, Arch. mikr. Anat. Iv, 1900. 



0. SCHULZE. Ueber das erste Auftreten der bilateralen Symmetric 

 im Verlauf der Entwicklung, Arch. mikr. Anat. Iv, 1900. 



2. MECHANICAL AGITATION 



The necessity of perpetual and violent agitation for the very 

 numerous pelagic ova which are ordinarily exposed to the stress 

 of wind and weather is well known to every zoologist who has 

 attempted to rear such forms in an aquarium, and need not be 

 further insisted on. 1 



There are also other eggs which require a small amount of 

 movement. The Hen turns her eggs every day, and the opera- 

 tion has to be artificially performed in an incubator. Its omission 

 leads to serious consequences, for, as Dareste has shown, the 

 allantois sticks to and ruptures the yolk-sac in unturned eggs, the 

 ruptured yolk-sac cannot be withdrawn into the abdomen, and 

 the Chick cannot hatch out. Death may ensue at an early stage. 



A violent agitation of the Hen's egg, on the other hand, is 

 equally fatal. 



Dareste subjected the unincubated eggs to violent shocks at 

 the rate of 27 a second for varying periods (from hour to 

 1 hour). The percentage of monstrosities observed after three 

 or four days of incubation was very high indeed, except when 



1 It seems probable that the principal value of the mechanical agita- 

 tion to the larvae is to prevent the Diatoms and Algae, of which their 

 food consists, from sinking to the bottom. 



