244 ROLE OF CYTOPLASM [CH. 



characters do not necessarily prove this conclusion, there are 

 a number of observations which indicate that the cytoplasm 

 is not a negligible factor. 



In the first place it is clear that the quite early stages of 

 development, including cleavage and the appearance of 

 embryonic symmetry, are in most animals, perhaps in all, 

 determined by the egg, and from their correspondence with 

 the egg-structure, almost certainly by its cytoplasm. All 

 eggs show a definite polarity, corresponding with the an- 

 terior and posterior ends of the future larvae, and in several 

 groups of animals, such as Insects, the egg also has a bilateral 

 structure, which, quite apart from the spermatozoon, deter- 

 mines the orientation of the larva to which it gives rise. Not 

 infrequently, also, the egg contains areas or zones of differ- 

 ent substances and a study of the development shows that 

 these become distributed quite regularly to definite organs 

 or tissues, and although the experimental rearrangement of 

 these visible substances (as by centrifuging) may show that 

 the orientation of parts in the larva depends on the polarity 

 of the egg as a whole rather than on the specific substances, 

 it seems indubitable that the process of segmentation and 

 differentiation of the parts of the embryo in many species 

 is conditioned by the egg-cytoplasm and influenced scarcely, 

 if at all, by the spermatozoon. It is of course not easy to 

 devise an experiment that shall prove this directly, for the 

 type of cleavage is in general characteristic of large groups, 

 and so it is hardly possible to cross species of different types 

 in order to see whether the cleavage of the hybrid is different 

 from that of the female parent's species. 



About the rather later stages of development there has 

 been some difference of opinion. It is said by several investi- 

 gators, for example DRIESCH, that in Echinoids the number 

 and mode of formation of the primary mesenchyme cells 



