JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKIL 9 
D. Differentiation during Oleavage. 
Besides these questions in regard to the form and rate of cleavage, 
we have also the question of the qualitative nature of cleavage. ls 
cleavage merely a quantitative process, or is it accompanied by a 
differentiation of the separated cells? And if the latter is the case, by 
what means is this differentiation 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 aecompanying 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 während der Furchung gern 
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 duo, 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). 
9. 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 caso of the sea-urchin, the manner of cleavage is entirely 
