320 THEORIES OF INHERITANCE AND DEVEIOPMENT 



cases it may segment like an entire ovum of half-size (Fig. 132, D) and 

 give rise to an entire blastula.^ We may interpret this to mean that 

 in AmphioxiLS the differentiation of the cytoplasmic substance is at 

 first very slight, or readily alterable, so that the isolated blastomere, 

 as a rule, reverts at once to the condition of the entire ovum. In the 

 sea-urchin, the initial differentiations are more extensive or more 

 firmly established, so that only exceptionally can they be altered. In 

 the snail we have the opposite extreme to Amphioxus, the cytoplasmic 

 conditions having been so firmly established that they cannot be 

 altered, and the development must, from the outset, proceed within 

 the limits thus set up. 



Through this conclusion we reconcile, as I believe, the theories of 

 cytoplasmic localization and mosaic development with the hypothesis 

 of cytoplasmic isotropy. Primarily the egg-cytoplasm is isotropic in 

 the sense that its various regions stand in no fixed and necessary rela- 

 tion with the parts to which they respectively give rise. Secondarily, 

 however, it may undergo differentiations through which it acquires a 

 definite regional predetermination which becomes ever more firmly 

 established as development advances. This process does not, how- 

 ever, begin at the same time, or proceed at the same rate in all eggs. 

 Hence the eggs of different animals may vary widely in this regard 

 at the time cleavage begins, and hence may differ as widely in their 

 power of response to changed conditions. 



The origin of the cytoplasmic differentiations existing at the be- 

 ginning of cleavage has already been considered (p. 285). If the 

 conclusions there reached be placed beside the above, we reach the 

 following conception. The primary determining cause of develop- 

 ment lies in the nucleus, which operates by setting up a continuous 

 series of specific metabolic changes in the cytoplasm. This process 

 begins during ovarian growth, establishing the external form of the 

 ^gg, its primary polarity, and the distribution of substances within it. 

 The cytoplasmic differentiations thus set up form as it were a frame- 

 work within which the subsequent operations take place, in a more 

 or less fixed course, and which itself becomes ever more complex as 

 development goes forward. If the cytoplasmic conditions be artifi- 

 cially altered by isolation or other disturbance of the blastomeres, a 

 readjustment may take place and development may be correspond- 

 ingly altered. Whether such a readjustment is possible, depends on 

 secondary factors — the extent of the primary differentiations, the 

 physical consistency of the egg-substance, the susceptibility of the 

 protoplasm to injury, and doubtless a multitude of others. 



1 I have observed this only twice. In both cases the cleavage up to the sixteen-cell stage 

 was exactly like that of the entire egg except that the micromeres were relatively larger, as 

 shown in the figure. 



