154 J- I r - MCCLENDON. 



action, which would account for the secretion of urine against a 

 concentration gradient, but a simpler mechanism of such a process 

 is shown in Hamburger's double membranes. 



The blood pressure may aid in the secretion of the water of 

 the urine, which is eliminated chiefly through the glomertili, 

 but its insignificance in the elimination of urea is shown by the 

 fact that after increasing the volume (and therefore pressure) of 

 rabbit's blood 70 per cent, by transfusion, the urea elimination 

 was not or only very slightly increased. 



VII. CELL DIVISION. 



Various hypotheses as to the cause of cell division have been 

 advanced by the morphologists. Hertwig, supposed that when 

 the ratio of nucleus to cytoplasm is less than normal, the cell 

 will divide. 1 Gerassimow 2 subjected cells of Spirogyra to low 

 temperatures and other abnormal conditions and obtained an 

 increased amount of chromatin in some of them. These cells 

 did not divide until the ratio of nucleus to cytoplasm was as 

 great as at the time of division of a normal cell. 



I found that chromatin is not necessary for cell division. 3 

 After extracting the chromosomes from the starfish egg, I caused 

 it to divide. In this case the ratio of nucleus to cytoplasm was 

 zero; however the cell did not continue to divide indefinitely. 



There is no easy method of determining the ratio of nucleus 

 to cytoplasm. Some cells contain large vacuoles \vhose contents 

 are not considered as cytoplasm. Eggs contain fat drops and 

 granules compounded of protein and lipoids. These are not 

 considered as cytoplasm by all investigators. If the granules and 

 oil are included as cytoplasm, the ratio of nucleus to cytoplasm 

 is very small, and yet the egg cell does not divide unless "stimu- 

 lated" by the sperm or some other means. 



k. Lillie 4 observed that chemical substances, which in low 

 concentration cause the Arbacia egg to divide, in high concen- 

 tration cause outward diffusion of the red pigment (echinochrome) 

 and compared this to the laking of erythrocytes. 



1 lie is not confirmed by Conklin, Jour. Expcr. Zoo!.. 1912, XII.. i. 

 - Bull. Soc. Imp. Nat., Moskau, I<;<M. No. i. 



3 McClendon, Arch. f. Entwicklungsmcch., 1908, XXVI, 662. 



4 Bioi.. BULL., 1909, XVII., 188. 



