24 



INTRODUCTION TO CYTOLOGY 



nuclei which happen to enter the germinal region become germinal nuclei, 

 the rest becoming somatic nuclei (Huettner, 1923; on Drosophila). In 

 many such eggs the cleavage is at first only superficial, the walls forming 

 without special reference to the nuclei or their division, and the resulting 

 compartments remaining for a time open into the common underlying 

 mass of protoplasm. Many of the special tissues of the animal body may 

 begin their development as plasmodial masses in which cellular differentia- 

 tion occurs later. In some tissues the non-cellular condition may remain 

 until maturity, as in certain "syncytial" types of connective tissue. The 

 internal structures characterizing many tissue cells, such as neurofibrils 

 and elastic fibrils, may arise before the subdivision into cells (see Rohde, 

 1908, 1923). 



D 



Fig. 13. — Early stages in the embryogeny of Agathis australis. A, ccenocytic stage. 

 B, 32-nucleate stage with cytoplasmic suspensor cap differentiated. C, proembryo sub- 

 divided into cells. Three regions distinguishable: suspensor initials above, embryo 

 proper in middle, and embryonic cap below. D, later stage; cap well developed and 

 suspensor elongating. {After Eames, 1913.) 



All of this is not to be interpreted as a denial of the importance of 

 cells in multicellar organisms but only as a recognition of the fact that the 

 principle of functional protoplasmic differentiation is more general and 

 fundamental than that of cells as units (Heidenhain, 1907). Although 

 we thus look upon cellular organization as initially a result of differentia- 

 tion, it is true that it has, in turn, conditioned differentiation of a higher 

 degree. The presence of cell partitions allows a more effective segregation 

 of functionally specialized regions and a fuller play to those important 

 physico-chemical processes which depend on surfaces and thin films for 

 their action. Furthermore, it permits the development of larger plant 

 bodies by furnishing an ideal basis for the more effective operation of 

 turgor and for the deposition of supporting materials. The evolution of 

 higher organisms has unquestionably been very largely conditioned by the 

 multicellular state, but we should think of such organisms primarily as 

 highly differentiated protoplasmic individuals rather than cell republics. 



