GENES AND DEVELOPMENT 139 



known, the first steps of development can proceed even if nuclear 

 division is entirely suppressed. The only example of differential nuclear 

 division in the early stages of development is in Ascaris/ where it is 

 probably caused by, and not the cause of, cytoplasmic differences. 



The occurrence of differential divisions in later stages caimot be so 

 confidently denied. In plants and lower animals the possibility of 

 complete regeneration from small parts shows that they must contain 

 the whole set of hereditary factors. In higher animals there are a few 

 cases in which a region regenerates an organ other than the one removed 

 (homoesis). But usually the only evidence is the occurrence of small 

 mosaic patches due to somatic mutation in late developmental stages, 

 and these can in the nature of the case only reveal the presence of 

 those genes which affect the mosaic organ, and which would have to 

 be assumed to be there on either hypothesis. 



2. General Concepts of Developmental Mechanics 



Development is a historical process and we can only understand it if 

 we begin at the beginning. The first-formed organs of an animal may 

 be developed in either of two ways: (i) a particular part of the egg is, 

 from the earhest stage in which we can investigate it, determined to 

 become the organ and does become it under any conditions in which it 

 can develop at all, or (2) this irrevocable determination takes place 

 later and can often be shown to depend on the interaction between the 

 part in question and some other nearby part of the egg. Development 

 of the former kind occurs in the so-called mosaic eggs, and the parts 

 which are determined to become particular organs are said to contain 

 organ-forming substances. A deeper insight into the processes involved 

 can be obtained from eggs which follow the second scheme. Here the 

 various regions of the egg are at first capable of being altered from their 

 normal course of development; they are "indifferent" and may become 

 something other than their "presumptive fate." During a certain period, 

 a process takes place which fixes the future course of development of 

 the various regions; this process is "determination." The sort of organ 

 which any region is determined to become depends on its position 

 within the whole egg. When determination is complete, the egg is in 

 the same state as was the mosaic egg at the beginning of its develop- 

 ment; it consists of a set of regions, each determined to develop into an 

 organ of a certain kind, arranged in a definite pattern. Eggs of this 

 1 Boveri, Rev. Schleip 1929. 



