JENNINGS: DEVELOPxMENT OF ASPLANCHNA HERRICKII. 9 



D. Differentiation during Cleavage. 



Besides these questions in regard to the form and rate of cleavage, 

 ■we have also the question of the qualitative nature of cleavage. Is 

 cleavage merely a quantitative process, or is it accompanied by a 

 differentiation of the separated cells 1 And if the latter is the case, by 

 what means is this diflFerentiation accomplished ? 



The view once maintained, that cleavage is entirely unaccompanied by 

 differentiation of the separated cells, may be said to be nearly or entirely 

 given up ; the questions which remain relate to the means by which this 

 differentiation is brought about. In regard to this several well defined 

 views exist. 



1. Roux holds that the differentiation accompanying cleavage is a 

 result of qualitative karyokinesis ; i. e. at a given cell division the two 

 products receive nuclear material of different nature. 



2. Driesch maintains that the differentiation which may accompany 

 cleavage is due to the specific cytoplasmic structure of the egg, different 

 parts of the egg being of different constitution, so that when this 

 differentiated mass is separated into parts, these parts receive different 

 sorts of cytoplasm. That is, the qualitative division is in the cytoplasm, 

 not in the nuclear material. " Ich habe schon oben gesagt, dass ich ein 

 Verschiedenwerden der Furchungszellen wahrend der Furchung gem 

 zugebe, aber hierin nichts anderes als die Folge eines spezifischen 

 Plasmabaus des Eies sehe." (Driesch, '94, p. 100.) 



3. According to Wilson and Hertwig the differentiation accompanying 

 cleavage is due, largely at least, to the interaction of the blastomeres, 

 after division has taken place. This does not exclude the possibility of 

 the existence at the same time of a qualitative division of the cytoplasm, 

 as stated above (2). 



2. Later Developmental Processes. 



"With regard to the later developmental processes, it will not be 

 necessary to give here a review of the various factors and theories 

 which have been set forth by different authors. Driesch ('94) gives 

 an extended analysis of the morphogenetic process and its factors, 

 and Davenport ('95) presents a detailed list of the different processes 

 concerned in development. It is sufficient here to propose a single 

 question : What is the relation of the cleavage process to the secondary 

 morphogenetic processes ? Driesch's well known experiments indicate 

 that, in the case of the sea-urchin, the manner of cleavage is entirely 



