298 GENERAL EMBRYOLOGY 



germ cells, renders extremely' unlikely any strictly morpho- 

 logical conception of the relation between strictly differentiated 

 germinal determiners and the formation of certain tissues or 

 organs. The ideas cannot be overemphasized or repeated too 

 often that, while the thing or the relation that we call a deter- 

 miner may sometimes have a morphological expression in the 

 germ, essentially the relation is physiological functional 

 probably chemical or energetic (dynamic), and that the reac- 

 tions or interactions of whole groups and masses of particles or 

 systems are involved in determining the intermediate and final 

 results of development. 



One further group of observations must be considered in con- 

 nection with the possibility of the primary character of the 

 nuclear control of development and heredity. During the 

 process of development there occurs a constant giving off of 

 substance from the nucleus to the cytoplasm (Figs. 138, 32). 

 At every mitosis only a part of the chromatic substance is 

 formed into chromosomes, while the remainder passes into the 

 cytoplasm and dissolves, and of course the whole fluid content 

 of the nucleus, the nuclear sap, is discharged into the cytoplasm. 

 Herbst has emphasized the importance of the nuclear sap as an 

 important determining factor in development and heredity. 



In many instances, substances discharged from the nucleus 

 into the cytoplasm of the oogonia, especially during their 

 growth period, are directly concerned in the formation of specific 

 materials and bodies of the mature ovum. And later in develop- 

 ment Conklin has observed that the cilia of the superficial cells 

 of Crepidula develop only when certain chromatic granules 

 reach that region, a single cilium then differentiating opposite 

 each granule. We have already mentioned the fact, described 

 by Wilson, that in Cerebratulus the effects of the removal of 

 parts of the egg cytoplasm before the germinal vesicle has 

 broken down, are very different from the effects of the removal 

 of similar portions after the contents of the vesicle have been 

 discharged into the surrounding cytoplasm during the initial 

 stages of maturation. 



But the evidence for the hypothesis of nuclear determination 



