58 SEXUAL REPRODUCTION IN CERTAIN MILDEWS. 



nor to be confused with, the conception that the cell mechanism contains 

 definite and permanent parts with specific functions. It may even be 

 a question whether it is advisable to call such parts of the cell organs, 

 since they are not to be compared morphologically to the organs of 

 multicellular plants or animals. 



The most specific evidence which Boveri advances for his concep- 

 tion is the fact that the chromosomes grow and can thus be said to have 

 a youthful and an adult condition and that they reproduce by division. 

 These are interesting analogies with the cell itself; but the cytoplasm 

 as a mass also grows from the size it has in the daughter cell to its size 

 in the adult cell, and it is reproduced by division ; still, it adds nothing 

 to our understanding of the cytoplasm to call it an individual organism 

 in the sense in which we so characterize the cell. Reproduction by 

 division and growth are necessary characteristics of any even relatively 

 permanent portions of cells which assimilate, grow, and divide. It is 

 to be remembered, further, that the permanency of the chromosome 

 means only the continuity of a structure which is undergoing continu- 

 ally its own series of cyclic changes in resting-stage, mitosis, fusion, etc. 

 It is doubtless susceptible to minor alterations due to its changing enviT 

 ronment, and is an active seat of metabolic changes. And further, as 

 to the significance of such external features as permanence in number, 

 size, form, and position in the nucleus for the functions of the chromo- 

 somes in determining, through heredity, the structure and functions of 

 cells and cell colonies, we have as yet little positive evidence. It is not 

 impossible that the organization of the chromatin is a matter of molecu- 

 lar rather than a grosser structure. The doctrine of permanence of 

 the chromosomes as structures of the cell does not necessarily carry 

 with it the assumption that the chromosomes are themselves composed 

 of such differentiated structures, as is the cell. 



The evidence summarized by Boveri, while it is entirely convincing 

 as to the permanence of the chromosomes in the resting condition, is 

 almost wholly inferential and based on their appearance in constant 

 number, form, size, etc., in the division stages. Rosenberg (81) has 

 recently brought very interesting direct evidence that the chromosomes 

 are present as definitely differentiated structures in many nuclei in the 

 resting condition. He finds that in the resting nuclei of Capsella, Zos- 

 tera. Calendula, and other plants the resting nuclei show a series of 

 sharply differentiated masses of the same number as the chromosome 

 number for the species — 32 for Capsella, 12 for Zostera, 32 for Calen- 

 dula — and represent a form in which the chromosomes persist from one 



