302 Gene Action 



a large part of the individual; others occur later in development 

 and produce only small, localized effects. 



The development of an individual involves many complicated 

 chemical and physical interactions. The unfertilized egg consists 

 of a nucleus containing cytoplasm and one haploid set of ma- 

 ternal chromosomes. To this, on fertilization, are added a hap- 

 loid set of paternal chromosomes and perhaps a trace of paternal 

 cytoplasm. This fertilized egg is oriented in a certain position 

 and is in a given environment which is normal for that species. 

 This orientation may have a definite effect on development, sub- 

 jecting part of the egg to certain stimuli that the rest of the egg 

 does not receive. Whatever the cause, the early stages of devel- 

 opment of an embryo are usually a characteristic of a species 

 or a larger group. 



Let us take Crepidula, the slipper limpet, as an example. The 

 zygote divides into two large cells, which then divide to form a 

 quartet of four blastomeres. These cells divide to form four 

 cells much smaller in size, called micromeres. The large blasto- 

 meres are referred to as macromeres. Another quartet of micro- 

 meres is then formed from the macromeres. The first set of 

 micromeres divides so that the sixteen-cell stage consists of 

 twelve micromeres and four macromeres. The left posterior mac- 

 romere then divides to form a cell known as the 4D cell (Fig. 

 84). Successive divisions of the various cells proceed to form 

 the embryo. It is important to recognize that these successive 

 divisions always occur in a regular manner and that the various 

 groups of cells will develop into definite parts of the animals. 

 The four macromeres, exclusive of the 4D cell, form the entire 

 endoderm of the adult and the micromeres produce the ectoderm ; 

 from the 4D cell develops the entire mesoderm of the mature 

 animal. 



The earliest stages in the development of an animal seem to be 

 directed by the cytoplasm of the egg, which is, of course, purely 

 maternal in character. 



The complete development from the unfertilized egg to the 

 mature individual is an extremely complicated series of phe- 

 nomena even in those forms that we generally regard as "lower" 

 animals. There is considerable variation in the developmental 

 pattern in the various forms, but one thing stands out. Within 

 each species there is a certain pattern of development that is 



