322 GENERAL EMBRYOLOGY 



matin granules. And history, enumerating germ layers, cleav- 

 age cells, cytoplasmic organ-forming substances, chromosomes, 

 and chromioles (chromatin granules), warns us against the idea 

 that we must seek or hope to find ultimate particles, concrete, 

 definable, and representatively determinative in function. In 

 fact a few students of the problem frankly declare their belief 

 that the idea of any sort of representative particle mechanism 

 is futile, that the regulation of the processes of development 

 and heredity depends upon interrelations which are not sus- 

 ceptible of interpretation in terms of any material basis. 



Scientifically, however, we can to-day go no further back 

 than the chromosomes, for here we find the most fundamental 

 units whose actual behavior can be correlated with the facts 

 of development. To say scientifically, that the chromosomes 

 are (to-day known to be) the determining elements in develop- 

 ment and heredity, is not to deny the existence of other bodies 

 or conditions which may determine the existence and qualities 

 of the chromosomes. Granules we know, but of their behavior 

 we know little, and this little cannot at present be correlated 

 with the facts of development. Of the real existence of elements 

 underlying the granules we know nothing whatever. Assump- 

 tion of the reality of such bodies or conditions may be a logical 

 necessity, but to-day it carries us beyond the boundaries of 

 observed fact. 



To repeat a statement made on an earlier page, if the exist- 

 ence and activity of the chromosomes can be shown to be a 

 necessary link in the processes of development and heredity, 

 and if these can be shown to be the simplest and most nearly 

 primary factors whose behavior can be correlated with these 

 processes, then we shall be justified in saying that the chromo- 

 somes are to-day the determining factors in development and 

 heredity. 



REFERENCES TO LITERATURE 



BALTZER, F., Die Chromosomen von Strongylocentrotus lividus und 

 Echinus microtuberculatus. Arch. Zellf. 2. 1909. Ueber die 

 Beziehung zwischen dem Chromatin und der Entwicklung und 



