168 CHARLES R. STOCKARD 



fects become evident. Other individuals resume their cleavage 

 processes, form a typical blastoderm and begin the formation 

 of a germ-ring, which indicates the commencement of gastrula- 

 tion, but just here the degree of energy necessary for normal de- 

 velopmental processes is insufficient and a single embryonic bud 

 is not formed with that normal rate of growth which sup- 

 presses the appearance of other embryonic buds. Therefore, 

 instead of the one point proliferating at a disproportionate rate 

 to form the embryonic shield, two such points are established with 

 more or less equal rates of proliferation, both of which may be some- 

 what less active than the single one should be. The formation 

 of two embryonic shields or the initiation of two points of rapid 

 gastrulation away from which will grow the axes of the embryos 

 is in fact the initial or primary step in double formations. The 

 phenomenon is exactly the same as when two buds arise from two 

 notches on the leaf border instead of one bud growing from a 

 single notch. Every notch is a potential bud-forming point, and 

 in the same way many potential invagination points exist on the 

 blastoderm, and when more than one such place begins to grow 

 we have double formations. In this sense it may be appreciated 

 that the intrinsic conditions which give rise to double monsters 

 or twins exist in all eggs and are not produced by the experiment. 

 The experimental modifications of the external conditions simply 

 serve to allow more than the one growing point to express itself. 

 The actual results of several rather typical experiments may 

 be given as better illustrating the occurrence of the double 

 individuals. 



Experiment 903. A particularly fine lot of eggs was obtained from 

 a large female and fertilized by a single male on July 5, 1919, during 

 the height of the spawning season. Three groups of these eggs were 

 selected, one serving as a normal control and the two others, A: and A2, 

 at three and one-half hours after fertilization when dividing into the 

 8-cell stage, were placed in temperatures of 6° and 8°C., respectively. 



The outside temperature was unusually warm and the control eggs 

 developed at a vigorous rate. When 22 hours old the germ-rings were 

 from one-third to one-half over the yolk-spheres in all the specimens 

 the embryonic shields were well formed with the embryonic axes already 

 indicated in the midline. Ever}^ egg in the control lot was developing. 



