Influence of Environment 213 



2. Modifications During Fertilisation Stages. — Environmental 

 changes acting during fertilization may cause more than one sper- 

 matozoon to enter the egg or may injure the egg or sperm; in 

 either case the resulting development is abnormal. Where two or 

 more spermatozoa enter the egg the nuclear divisions are usually 

 abnormal, as Boveri has shown in the case of the sea urchin ; the 

 distribution of chromosomes to different cleavage cells is unequal 

 and such cells do not undergo typical development, while the 

 embryo or larva produced is not capable of continued life. In 

 cases where an egg is fertilized by a spermatozoon belonging to 

 a different phylum or class (heterogeneous fertilization) the 

 foreign sperm, after stimulating the egg to begin development, 

 may itself die or remain inactive, in which case the hereditary 

 traits which develop are those of the mother only. In many ani- 

 mals unfertilized eggs may be stimulated to begin development by 

 a great variety of changes in the medium, all such cases being 

 known as "artificial parthenogenesis." 



3. Modifications of Development after Fertilization. — Envi- 

 ronmental changes, acting upon the oosperm after fertilization, or 

 upon the embryo, may produce an almost infinite variety of ab- 

 normal types of development, but so far as known none of these 

 modifications becomes hereditary. It seems probable that changes 

 in hereditary constitution take place in the main before fertiliza- 

 tion and especially during the maturation divisions. 



Isolation of Cleavage Cells.— If the cleavage cells are separated 

 from one another in the 2-cell or 4-cell stage each of them may 

 give rise to an entire animal (Fig. 75) ; in this way two com- 

 plete animals may be derived from a single egg of a star-fish or 

 sea-urchin, of an amphioxus, and of several other animal types. 

 If the frog's egg is turned upside down in the 2-cell stage, double- 

 headed or double-bodied embryos may result (Fig. 76). In such 

 cases each cleavage cell is said to be totipotent, that is, it is ca- 

 pable of giving rise to an entire animal. 



On the other hand in certain animal phyla such as the cteno- 

 phores, mollusks, annelids and ascidians isolated cleavage cells 



