GENERATION OF ANIMALS, I\ . iv. 



since the fetations, on account of being situated 

 close to each other, grow on to each other, just 

 as many fruits sometimes do. Of these twin-eggs, 

 those in which the yolks are kept apart by the 

 membrane develop into two separate chicks, and 

 there is nothing extraordinary about them ; those in 

 which the yolks are continuous, with nothing to hold 

 them apart, give rise to chicks that are monstrosities : 

 they have one body and one head, but four legs and 

 wings, the reason for which is that the upper parts 

 of the body are formed out of the white and before 

 the rest, the nourishment being dispensed to them 

 from the store in the yolk, whereas the lower part 

 (a) is formed afterwards, (6) its nourishment is uni- 

 form and homogeneous.*^ 



A snake, too, has been seen with two heads, and 

 the cause is the same — this also is a class of animal 

 which lays eggs and is prolific. Monstrosities occur 

 less frequently, however, with snakes owing to the 

 shape of their uterus, in which, on account of its 

 length, the numerous eggs lie one after another in a 

 row.** Nothing of this kind occurs with bees and 

 wasps, because their offspring are laid in separate 

 cells. With the common fowl, however, the opposite 

 is the case — a fact which clearly goes to show that 

 we are bound to hold that the cause of such things 

 is in the material,'' since with other animals too 

 they occur more frequently in those that are prolific. 

 Hence they occur less frequently in human beings, for 

 the offspring which these produce is as a rule one in 

 number, and it is perfected by the time of birth, since 

 even in this species the occurrence of monstrosities is 

 more common in those regions where the women are 



' Not in the semen. 



423 



