The American Naturalist. 



[February, 



development : first, the increase of protoplasm ; second, the 

 progress of organization, i. e., of differentiation. 



As I was about to close this article, I received through the 

 kindness of the author, Nussbaum's address on differation, in 

 which he has defended essentially the same views as those 

 which I hold. Such an agreement is of great value to me. 



Now we know that larvte are animal forms which have to 

 obtain their own food and to protect themselves against ene- 

 mies, and therefore are provided with differentiated tissues. 

 Embryos, on the contrary, take their nutriment simply from 

 the ovum, and the cells continue for a long time, developing 

 and multiplying, while the protoplasm of the single cells in- 

 creases very slightly, and the beginning of the differentiation 

 proper is correspondingly postponed. I believe that we here 

 have to deal with causal relations. From the actual relations 

 just described, I conclude that the most essential difference 

 hitherto known between larvse and embryos, is to be found in 

 the differing lengths of the period of multiplication of undif- 

 ferentiated cells. In consequence of the shorter duration of 

 the period in larva* they have a much smaller total num- 

 ber of undifferentiated cells than embryos, or reversely ex- 

 pressed, embryos are much better equipped with material for 

 the construction of the adult body, than are larva-. As al- 

 ready stated, embryos are produced by the higher animals. 

 This fact finds its explanation in the relations just described 

 because the increased number of undifferentiated, or so called 

 embryonic cells, is precisely the necessary preliminary condi- 

 tion of the greater complexity of the differentiation by which 

 the animal becomes more highly organized. 



For the sake of clearness I have put aside all complications 

 which might come in to play. It goes without saying, that 

 the relations, in many respects, are by no means simple, nev- 

 ertheless, the main conclusion above given seems a secure 

 gain. 



I therefore interpret the embryo as a device to render possi- 

 ble the increase of undifferentiated cells, and consequently a 

 higher ultimate organization. The origin of this device is 

 conditioned by a supply of food independent of the embryo. 



