398 THE HEREDITARY FACTORS AND DIFFERENTIATION 



apparently, the asymmetry gradient with high point to the left. 

 These interact to form a complex compound system, no two points 

 in which will be in entirely identical conditions. 



It is these quantitative differences between regions of the embryo 

 which are responsible for initiating the processes of differentiation. 

 Of themselves, the hereditary factors are insufficient to account for 

 differentiation, and their action must be considered in relation to 

 the external factors and to the new internal factors which are con- 

 stantly arising as a result of antecedent processes of development : 

 internal factors which as such were not present in the undiffer- 

 entiated oocyte. 



A clear-cut example of the direct influence of the cytoplasmic 

 environment upon the chromosomes is furnished by the develop- 

 ment of Ascaris. Here, a process takes place known as the diminu- 

 tion of the chromatin, which occurs in all the blastomeres except 

 that one which will give rise to the reproductive organs. The fer- 

 tilised egg has normal chromosomes which divide at the first 

 cleavage, but in one of the resulting two blastomeres the ends of 

 the chromosomes are thrown off into the cytoplasm and their 

 middle portion breaks up into fragments. In the other blastomere 

 the chromosomes remain entire. In the subsequent divisions of the 

 blastomere with diminished chromosomes, all the chromosomes 

 appear in the diminished form. On the other hand, in the division 

 of the blastomere with entire chromosomes, one blastomere retains 

 the entire chromosomes, while those in the other blastomere under- 

 go diminution. A similar process occurs in the subsequent divi- 

 sions of the blastomere (always a single one) in which the chromo- 

 somes are entire, until it gives rise to the gonads (fig. 192). 



It has been shown by experiment that the presence in any blasto- 

 mere of the cytoplasm of the vegetative pole of the egg (containing 

 the so-called "brown granules") prevents the diminution of the 

 chromosomes. Normally, since the first cleavage division in Ascaris 

 is in the equatorial plane of the tgg, the division spindle being ver- 

 tical in the plane of the egg-axis, only one blastomere of the 2-cell 

 stage contains the vegetative-pole cytoplasm, and therefore only 

 one blastomere preserves the entire chromosomes. If a ripe tgg is 

 placed in a centrifuge apparatus and rotated at 3800 revolutions per 

 minute for several hours, the egg, being free to revolve, orientates 



