94 



Scientific Proceedings (98). 



periods of time in many species without noticeably injuring the 

 embryo. Such an interruption normally occurs during the de- 

 velopment of certain eggs as those of birds and some mammals. 

 In eggs having a continuous development, such as those of fish, 

 the rate of development may be slowed to apparent stoppage at 

 many stages and held in such a condition for some time without 

 injury to the embryo which results after the inhibiting influence 

 has been removed. However, when the rate of development is 

 retarded but not entirely stopped at certain critical periods and 

 development is allowed to proceed at this diminished rate for some 

 time, most serious structural anomalies are induced. 



Double monsters of varying degrees of doubleness may actually 

 be produced by slowing the rate at a time when the primary 

 embryonic bud should arise. Normally the initial appearance of 

 the primary embryonic bud probably suppresses the appearance 

 of other buds which potentially exist, but when the primary bud 

 is delayed in its appearance it becomes possible for more than one 

 bud to arise, usually two. The distance apart of these two buds 

 on the blastodisc determines the degree of doubleness of the result- 

 ing individual. Buds arising close together give two-headed 

 monsters, while buds arising at opposite points on the periphery 

 of the disk, 180 0 apart, each give rise to a complete individual, in 

 such a case twins result. 



When the two buds arise simultaneously they have equal 

 chances in development and symmetrical double monsters result. 

 If, however, one bud obtains the start over the other bud this 

 start constitutes a supremacy which almost invariably makes it 

 possible for the leading bud to develop into a perfectly normal 

 specimen, and invariably defeats the possibility of normal develop- 

 ment on the part of the slower bud. 



An investigation of a large series of such double fish embryos 

 lends strong support to the interpretation that the late bud is 

 inhibited in its rate of development on account of the presence of 

 the leading bud, just as the first bud to grow out from a notch on 

 the leaf of Bryophyllum inhibits the growth of other buds as Loeb 

 has so strikingly shown. The inhibited rate of development in 

 the lesser component tends to suppress and interfere with the 

 normal origin and development of certain organs, especially the 



