CHROMOSOMES AND CYTOPLASM IN PROTOZOA 



45 



Cytoplasm and chromosomes thus are not 

 separate in fact and in substance, though 

 they are often kept isolated in concept — to 

 the great detriment of sound understand- 

 ing. There is a continual cycle of inter- 

 change between them, a transformation of 

 material from one to the other. Such a 

 cycle recurs at every cell generation. As 

 Conklin (1902) remarks in summarizing his 

 illuminating observations on these phe- 

 nomena in Crepidula, "One might speak 

 of these changes in the nucleus as systole 

 and diastole, by means of which an ex- 

 change of nuclear and cytoplasmic mate- 

 rials is brought about." 



The observations made with the micro- 

 scope must be supplemented by considera- 

 tions drawn from the discoveries of genetics 

 and of experimental embryology. We know 

 from genetics that the chromosomes are 

 effective in determining the characteristics 

 of organisms. We know that they cannot 

 determine the developed characteristics 

 without influencing, altering, determining, 

 the processes of development by which the 

 characteristics arise. We see them doing 

 this in the processes that have just been 

 illustrated. Bodies as demonstrably active 

 as the chromosomes, and effective in such 

 diversified ways, must greatly alter the 

 materials that they absorb from the cyto- 

 plasm ; they must give off greatly changed 

 materials into the cytoplasm. Observation 

 in favorable organisms of what occurs in 

 the earliest stages of development is 

 strongly in agreement with this conclusion. 

 Let us recall a few such cases. 



The absorption of cytoplasmic materials 

 into the nuclear vesicles is most conspicuous 

 in the egg at the very beginning of develop- 

 ment, just before cell division begins. So 

 much material is absorbed that the nucleus 

 becomes enormously enlarged, forming the 

 great ''germinal vesicle," which may have 

 half or more the diameter of the entire egg. 

 Then by the solution of the nuclear mem- 

 brane all this absorbed and altered material 

 is cast back into the cytoplasm. And there- 

 upon occur revolutionary changes in the 

 egg cytoplasm. Its materials rearrange 

 themselves, become stratified, and take on 



an organization which forms the basis for 

 the organization of the young animal. 

 These changes have been vividly presented 

 in the sea urchin by Boveri, and in the 

 ascidian by Conklin. In this fundamental 

 organization, those materials which have 

 been elaborated in the chromosomal vesicles 

 visibly play a conspicuous and important 

 role. 



The further course of development, after 

 this fundamental organization has been laid 

 down in the undivided egg, is such as to 

 suggest — nay, perhaps to disclose — an out- 

 line schema of the role of chromosomes and 

 cytoplasm in the origin of bodily differen- 

 tiations. The organization of the ovum 

 shows itself in the stratification of different 

 materials in the so-called ' * egg pattern ' ' : 

 a stratification which shows that there must 

 have been some fundamental axial differen- 

 tiations in the cytoplasm, handed on from 

 the parental cytoplasm before the stratifica- 

 tion took place. By cell divisions the dif- 

 ferent formative materials of the diverse 

 layers are separated into different cells. 

 And these cells containing diverse cyto- 

 plasmic materials develop differently, to 

 produce different parts of the young indi- 

 vidual. And differentiation in accordance 

 with this schema is progressive. Parts that 

 at first form a homogeneous unity later 

 become differentiated into diverse struc- 

 tures by an extension of these processes. 



In bringing about these differentiations, 

 there is repeated in every cell the "systole 

 and diastole" of chromosomal and cytoplas- 

 mic interchange. But by the earlier steps 

 of this interchange, the cytoplasms of the 

 different cells have already become diverse. 

 Therefore the reactions w^ith the chromo- 

 somes must now give different products in 

 the different cells. 



Further, the chromosomes, as we know, 

 are not homogeneous; each single chromo- 

 some is composed of many different mate- 

 rials commonly known as genes, and these 

 materials have very diverse effects in devel- 

 opment. The different cytoplasmic mate- 

 rials in the different cells must react in 

 different cases with different chromosomal 

 materials, yielding diverse products in the 



