mental than other structures on which relationship is based. Among the 

 most rewarding studies in this regard are detailed comparisons of karyo- 

 types in apparently related groups, analysis of meiosis, especially in 

 putative hybrids, and studies of natural and synthetic polyploids. Studies 

 on the origin of important crop plants such as wheat, cotton, and to- 

 bacco have drawn heavily on such cytological and cytogenetical studies, 

 as have the evolutionary relationships of species of Crepis worked out 

 over many years by E. B. Babock and his colleagues. Similar studies 

 have been made on animals, especially on insects and other invertebrates, 

 as described by M. J. D. White. 



Embryology 



We may define embryology (plant or animal) as the study of processes 

 associated with the early stages in the development of an individual from 

 a functional Qgg. As we examine any standard textbook on the subject, 

 it becomes clear very quickly that most of the problems of cellular biol- 

 ogy are also problems of developmental biology. Embryology must con- 

 cern itself with a level of organization generally considered to be higher 

 than that which occupies the attention of the cytologist; namely, cellular 

 interrelationship in the development of tissues and organs. Nonetheless, 

 this major question of embryology demands equal and prior concern for 

 the properties of cells themselves. The problem of cellular replication is 

 a vital one to the student of growth and development, since he is faced 

 with the problem of control of this phenomenon in relation to progressive 

 organization. By the same token the embryologist must take full cogni- 

 zance of the basic structure of cells, including the role, distribution, 

 origin, and fate of particulate fractions. Whatever may be the specific 

 factors involved in differentiation of cells, tissues, and organs in any par- 

 ticular case, we must look for both the structural organization and bio- 

 chemical capacities which enable a cell to respond in a specific way and 

 to the extracellular conditions which induce the response. It should be 

 noted here that the extracellular conditions which induce change in one 

 kind of cell may well be the intracellular conditions in another type of 

 cell. In many cases the only thing that distinguishes between an em- 

 bryologist and a cytologist is some slight difference in the point of view 

 from which they attack the same problems. 



While there is no doubt that full comprehension of the "grand plan" 

 of the cell would place our understanding of the organic world on a par 

 with that of the inorganic, it would not automatically solve all biological 

 problems. Such knowledge would be a major step in the right direction, 

 but profound questions concerning the integration of cells into tissues, 



274 / CHAPTER 12 



