STRUCTURE AND DEVELOPMENTAL RATE 197 



e. ^Alternation of generations' and twins in vertebrates 



Among plants and lower animals, particularly the coelenterates, 

 there commonly exists a so-called alternation of generations. A 

 given species at one time reproduces sexually by the union of 

 gametes, egg and sperm cells, and the individuals derived from 

 such gametes then give rise to a number of other individuals by 

 a growth and fission or a budding process. Finally, sexually 

 mature individuals again occur to reproduce another generation 

 from germ-cells. In general this phenomenon is thought to be 

 limited to these lower forms. 



The suggestion has frequently been made but without sufficient 

 emphasis that the blastoderm may be looked upon as a stock 

 able to give rise asexually to more than one embryo. Since the 

 natural process of budding to form four or more embryos in 

 the armadillo is recognized, and accessory individuals may be 

 produced experimentally from other vertebrate eggs, it becomes 

 evident that even man and the highest animals may actually at 

 times exhibit an alternation of the sexual and asexual processes of 

 reproduction. 



In a subsequent section of this paper the origin of various or- 

 gans of the individual's body will be considered as arising initi- 

 ally through a budding process exactly comparable to the ini- 

 tial embryonic axis bud on the blastoderm. These buds may 

 also be suppressed or inhibited in their expression in much the 

 same way and by similar experimental methods as was described 

 above in the case of the embryonic axis or initial embryo bud. 



From a general biological standpoint the adult body of higher 

 animals may be very correctly considered to be derived from a 

 sexually produced embryonic axis the stock which gives rise by 

 an asexual method of budding to the various special organs. 

 The vertebrate body is thus composed of a group of different 

 zooids, the organs. There are seeing, hearing, excretory zooids, 

 and so on, comparable to the zooids of a siphonophore colony. 



Alternation of generations is here considered a phenomenon, 

 not limited as is generally taught to lower forms, but occurring 

 throughout the animal kingdom. 



