THE PROBLEMS OF DEVELOPMENT 7 



of parts of an animal can be prevented by irradiation, although such irradia- 

 tion has no injurious effect on the animal as a whole. Under normal condi- 

 tions the amputated limb of a salamander will regenerate the parts which 

 have been removed, but if the animal is irradiated with X rays this regenera- 

 tion is blocked. 



To continue the survey of the problems, the dividing egg gradually trans- 

 forms into an embryo with structures such as brain, eye, heart, muscle, liver. 

 What factors determine which cells of the dividing egg will become eye and 

 which cells heart? Are all the parts of the adult represented in miniature in 

 the egg, as the preformation theory held? This situation is certainly not 

 true within the range of microscopic vision. But perhaps on a chemical basis 

 there may be some kind of chemical preformation not visible by microscopic 

 methods. In this event other methods must be used to determine whether the 

 structures of the adult are preformed in the egg. 



For example, it is possible to separate one egg into two halves and observe 

 each of these halves develop into a complete embryo. This is now a classical 

 experiment, done many years ago, but it still has great significance in this 

 connection, because from a single egg two complete individuals may be 

 obtained. Some eggs are so large that a ligature, or a knot, can be tied 

 through the middle of them. If this knot is tied rather tightly, the two 

 halves will begin to develop somewhat independently, resulting in embryos 

 like those pictured in Figure 2, with two complete heads, two eyes in each 

 head, and a complete duplication of limbs, gills, heart, and many other struc- 

 tures. Internally these embryos show all parts of the brain present in duplicate. 



If the ligature is tied still more tightly the embryo is cut completely in 

 two and two complete embryos may be obtained from a single egg. In the 

 eggs of some forms even the four quarters will produce four complete 

 embryos (Fig. 3). Conversely, two eggs can be fused together and they will 

 form a single embryo. Thus, for example, at the two-cell stage two eggs are 

 fused together (Fig. 4). The dumbbell-shaped structure AB is one egg; CD 

 is another. If these two dumbbells are put together — with a little pressure — - 

 they will fuse to form a four-cell stage. The fusion mass is irregular for a 

 while, but it soon rounds up, forms a compact mass which develops a single 

 nervous system, and finally goes on to form an embryo in which the parts 

 are all single. Here, then, the parts of two eggs put together fuse perfectly 

 to form a single embryo. 



Clearly the various parts of an embryo are not completely fixed in the egg, 



