SUSCEPTIBILIIY OF AMPHIBIAN OVA TO X-RAYS 



447 



injured in such a way that localized lesions are produced in the 

 larva. In rare instances the entire side of the body may fail to 

 develop (hemi-embryo.) This suggests the possibility that at 

 times the injured paternal chromosomes may be localized in one 

 of the first two blastomeres, while the normal maternal chromo- 

 somes are localized in the other blastomere and are capable of 

 inducing differentiation up to the larval stage. 



In this connection the various experiments made on crossing 

 different species and genera of amphibians are of interest. Ziegler 

 ('02), has summarized the results of Pfliiger, Born, Gebhardt 

 and others as follows: 



Rana esculenta, 9 \ 



Rana fusca, cf / The eggs develop to the blastula stage. 



Owing possibly to the shape of the head of the spermatozoon of R. esculenta 

 crossing in the reverse direction does not lead to fertilization. 



\ The eggs develop into larvse, some of which meta- 

 / morphose into frogs. 



No fertilization in the reverse direction. 

 Development continues up to gastrulation; in 

 favorable cases up to the origin of the medullary 

 plate. The cleavage stages appear normal but 

 during gastrulation development becomes very 

 abnormal. 

 \ The eggs segment and develop to the morula 

 / stage. 



As a rule no fertilizations take place in the reverse 

 direction, but one of 100 eggs one segmented 

 regularly and two irregularly. 

 \ The eggs develop to larvse, and these meta- 

 J morphose into toads. 

 Bataillon [Comptes rendus de I'Acad. des Sciences, 1908,] has crossed sev- 

 eral species of amphibia which have a varying number of chromosomes in reduc- 

 tion division. The species used together with the estimated number of chromo- 

 somes were as follows: Pelodytes punctatus, 6; Bufo calamita, 12; Bufo vulgaris, 

 8-9; Rana fusca, 12. 



1. Some not fertilized. 



2. Some underwent parthenogenetic segmen- 

 tation. 



3. Some ceased developing in the blastula stage. 

 Blastula halves characterized by 2 kinds of cells; 

 (1) Large with large nuclei (12 chromosomes), 

 and (2) small with small nuclei (6 chromosomes) 



J 4. About 10% developed into larvse. 



Rana arvalis, 9 

 Rana fusca, cf 



Rana arvalis, 9 

 Rana esculenta, cf 



or 

 Rana arvalis, cf 

 Rana esculenta, 9 

 Bufo vulgaris, 9 

 Rana fusca, cf 



Bufo cinerius, 9 , 

 Bufo variabilis, cf 



Pylodytes punctatus, <f 

 Bufo vulgaris, 9 



