FERTILISATION 205 



the material upon which the sperm operated.^ Seehger,"^ Morgan/' 

 and others have objected to Boveri's conclusion on the ground that 

 larviE arising from cross-fertilisation show an unusually wide range of 

 variation. Moreover, Godlewsky * has carried out an experiment in 

 which he fertilised a non-nucleated portion of a sea-urchin's egg with 

 the spermatozoon of a crinoid, and obtained, as a result, a larva of 

 the maternal type. This experiment, if correctly described, seems 

 to nullify Boveri's conclusion. 



Hickson h&s remarked that if it be true that the chromosomes 

 are the sole carriers of heredity it seems to be necessary to believe 

 in the individuality of the c-firomosomes ; that is to say, that the 

 chromosomes seen at the poles of the spindle at the termination of 

 mitosis are individually identical with those seen at the equator of 

 the spindle at the next mitosis. He points out, further, that there 

 is distinct evidence that this is not the case in certain Protozoa and 

 Coelenterata. Again, Hickson has called attention to the long 

 duration of the period of conjugation in Infusoria (Heterokaryota), 

 remarking that this is difficult to explain if we accept the view that 

 the cytoplasm of the conjugating cells is not concerned with the 

 transmission of hereditary characters.^ 



On the other hand, the persistence of parental chromosome groups 

 after fertilisation in many animals, and also the evidence of hetero- 

 genous hybridisation experiments, lend considerable support to tlie 

 theory of the individuality of the chromosomes, which gains more 

 ground every year. In Cyclops,^ Crepidula^ Crypt6branchn,s^ the two 

 parental groups of chromosomes after fertilisation are clearly distin- 

 guishable by their size and shape. Their peculiar characteristics are 

 retained through a number of successive cell divisions, the two sets 

 of chromosomes being always distinguishable on the equatorial plate 

 of the mitotic spindle. In Echinus, where the chromosomes number 



1 The nuclei of such larvae have been shown to possess only half the normal 

 number of chromosomes ; see Morgan, " The Fertilisation of Non-nucleated 

 Fragments of Echinoderm Eggs," Aroh.f. Entwich.-Mechanik, vol. ii., 1895. 



2 Seeliger, " Giebt es Geschlechtlicherzeugte Organismen ohne Miitterliche 

 Eigenschaf ten ? " Arch. f. Entwick.-Mechanih, vol. i., 1894. 



^ Morgan, loc. cit. See also Wilson, loo. cit. 



* Godlewsky, " Untersuchungen uber die Bastardierung der Echiniden und 

 Crinoiden-Familie," Arch. f. Entwick.-Mechanik, vol. xx., 1906. 



* Hickson, ".The Physical Basis of Inheritance," British Assoc. Reports, 

 Leicester Meeting, 1907, and Trails. Manchester Micr. Soc, 1907. See also Tick, 

 "Vererbungsfragen, Reduktions- und Chromosomenhypot|jesen," Merkel und 

 Bonnets Ergeh. f. Anat. u. Phys., vol. xvi., 1906. > 



•^ Haeckerj Ueber die Selbstandigkeit der vaterlichen und miitterlichen 

 Kernbestandteile wahrend der Embryonalentwicklung von Cyclops," Arch. f. 

 Mikr. Anat., vol. xlvi., 1895. 



' Conklin, "The Embryology of Crepidvla," Jour. Morph., vol. xiii., 

 1897. 



8 Smith (B. G.), "The Individuality of the Germ Nuclei during the Cleavage 

 of the Egg of CryptobranchiK," Biol. Bull., vol. xxxvii., 1919. 



