THEORIES OF POI^YEMBRYONIC DEVELOPMENT 93 



with Assheton in his objection to the budding hypothesis 

 and in considering the process by which the individual 

 embryonic primordia emancipate themselves from the 

 common germ as a process oi fission, or dividing up of 

 the ectodermic vesicle into several ectodermic primordia, 

 each of which stands for an embryonic area. The part 

 that is left behind, the common amnion, is a compara- 

 tively insignificant residue and could scarcely be termed 

 the stock. 



CONCLUSION 



In reviewing all of the facts it now seems to me that 

 the position that there is a genetic connection between 

 the four embryos and the four blastomeres is entirely 

 untenable. The extraordinarily irregular arrangement 

 and number of fetuses in D. hybridus seem to be totally 

 out of accord with this idea. The budding theory, 

 though affording a convenient descriptive device, 

 appears to be open to serious objection, as shown by 

 Assheton, yet the mechanism of outgrowth production is 

 not unlike certain true cases of budding. The fission 

 idea seems to be on the whole less open to criticism, if 

 by fission we mean merely the physiological isolation 

 of several secondary points in a single embryonic 

 vesicle, and the consequent acquisition by these points 

 of independence in growth and development. 



Unquestionably long before the isolation of several 

 secondary growing points a considerable amount of 

 differentiation has occurred, so that genetic factors are 

 unequally distributed in the various regions which give 

 rise to the new apical points. We may expect a less 

 degree of difference between closely adjacent points 



