2o6 THE PHYSIOLOGY OF REPRODUCTION 



about, thirty-six, many of these have a distinctive shape in the form 

 of long or short pot-hooks, chibbed or V-shaped rods. According to 

 Baltzar/ the same chromosomes can be identified over and over again 

 in successive segmentation divisions, by their peculiar shape. In 

 hybrid fishes obtained by crossing the salt-water minnow Fundulils 

 with the silver-sided minnow Menidia, Moenkhaus ^ could distinguish 

 the long chromosomes of Fundulus (which are about 2'18 /x) from the 

 short ones of Menidia (which are only 1 /* in length) in all the 

 segmentation divisions. Morris^ even claims that in the*cross between 

 Fundulus and the wrasse Ctenolabrus, the paternal and maternal 

 portion. of the chromatin can be clearly distinguished in the resting 

 condition of the nuclei. In many other forms, as Sakamura* has 

 recently shown in Vicia, certain of the chromosomes have definite 

 constrictions near their end and sometimes in the middle; these 

 constrictions appear in exactly the same position on these chromo- 

 somes in all successive cell divisions. 



Verworn has objected on general grounds to the doctrine that 

 the hereditary transmission of parental characteristics is mediated 

 by the transference of nuclear substance only. This is what he 

 says: "The physiological mode of thought will hardly be able to 

 adapt itself to the idea of a single hereditary, sw&stewce, which is 

 localised somewhere in the cell, and transferred in reproduction. 

 A substance that is to convey the characteristics of a cell to its 

 descendants, before all else must be capable of . life, i.e. must have a 

 metabolism, and this is impossible without a connection with other 

 substances necessary to cell-metabolism, i.e. without the integrity of 

 all essential cell-constituents. The 'designation of a single cell- 

 constituent as the specially differentiated bearer of heredity is wholly 

 unjustified ; the cell protoplasm is of exactly the same value in this 

 respect as the nucleus, and we must constantly return to the fact 

 that in all living nature no instance is known in which a complete 

 cell possessing nucleus and protoplasm does not always mediate 

 hereditary transmission. The charapter of every cell is determined 

 by its peculiar metabolism. Hence, if the peculiarities of a cell are 

 to be transmitted, its characteristic metabolism must be transmitted • 

 this is only conceivable when nuclear substance and protoplasm, with 



' Baltzar, "Die Chromosomen von Stronglocentrotus lividm und Echinus 

 microtuhercidatua" ^rch. f. Zellforsch.,.\o\. ii., 1909. 



^ Moenkhaus, "The Development of the Hybrids between Fwadulm 

 heteroditus and Menidia notatus, with especial reference to the Behaviour of 

 Maternal and Paternal Chromosomes," Jow. Anat., vol. iii., 1904. 



5 Morris (M.), " The Behaviour of the . Chromatin in Hybrids between 

 Fundulus and Ctenolabrus," Jour. Exp. Zool., xvi., 1914. 



* Sakamura, " Experimentelle Studien iiber die Zell- und Kernteilung 

 mit besonderer Eiicksicht auf Form, Grosse, und Zahl der Chromosomen," 

 Jow. Coll. Science, Imp. Univ. Tokyo vol. xxxix., 1920. 



