Influence of Environment on Animals 225 



the opinion that the formation of the blastula may be caused 

 general^ by a tropic reaction of the blastomeres, the latter 

 being forced by an outside influence to creep to the surface of 

 the egg. 



These examples maj^ suffice to indicate that the arrangement 

 of definite groups of cells and the morphological effects resulting 

 therefrom may be determined by forces lying outside the cells. 

 Since these forces are ubiquitous and constant it appears as if 

 we were dealing exclusively with the influence of a gamete; 

 while in reality all that is necessary for the gamete to transmit 

 is a certain form of irritability. 



d) Factors which determine place and time for the deposition 

 of eggs. — For the preservation of species the instinct of animals 

 to lay their eggs in places in which the young larvae find their 

 food and can develop is of paramount importance. A simple 

 example of this instinct is the fact that the common fly lays 

 its eggs on putrid material which serves as food for the young 

 larvae. AVhen a piece of meat and of fat of the same animal are 

 placed side by side, the fly will deposit its eggs upon the meat 

 on which the larvae can grow, and not upon the fat, on which 

 they would starve. Here we are dealing with the effect of a 

 volatile nitrogenous substance which reflexly causes the peri- 

 staltic motions for the laying of the egg in the female fly. 



Kammerer has investigated the conditions for the laj^ng of 

 eggs in two forms of salamanders, e.g., Salatnandra atra and 

 S. maculosa. In both forms the eggs are fertilized in the body 

 and begin to develop in the uterus. Smce there is room only 

 for a few larvae in the uterus, a large number of eggs perish 

 and this number is the greater the longer the period of gestation. 

 It thus happens that when the animals retain their eggs a long 

 time, very few young ones are born; and these are in a rather 

 advanced stage of development, owing to the long time which 

 elapsed since they were fertilized. When the animal lays its eggs 

 comparatively soon after copulation, man}- eggs (from twelve to 



