36 HEREDITARY CHARACTERS 



Recent experiments in hybridisation also seemed to throw 

 quite a different light upon the influence of the nuclei in 

 fertilisation. 



Experiments performed by Driesch 1 and Boveri 2 (1903) 

 showed that in rapidity of segmentation and general 

 appearance, hybrid embryos possessed pure maternal 

 characters. 3 



Loeb fertilised sea-urchins' eggs with the sperm of the 

 star-fish, and the resulting larvae exhibited pure maternal 

 characters. 



Many interesting experiments have been made in the 

 way of extirpating various parts of the eggs of different 

 animals, which seem to throw a good deal more upon the 

 cytoplasm than was generally admitted only a few years ago. 



Fischer removed from unsegmented Ctenophore eggs a 

 certain portion of the yolk, and showed that in the resulting 

 larvae one or more of the ctenidia are not developed. 4 He 

 also found that when certain parts of the ovum were re- 

 moved, other parts of the larvae failed to develop. Similar 

 experiments with similar results were made by Wilson with 

 the eggs of Patella and Dentalium. 6 



there were a layer of cells upon a ball. The next stage is that the surface of 

 the ball is pushed in at one point. This goes on until quite a large hollow is 

 produced, lined inside by cells derived from the outer layer, just as though the 

 surface of an indiarubber ball had been pushed in at one point with the finger. 

 At first the mouth of the hollow is large. It gradually narrows, the space 

 inside is completely cut on", and eventually gives rise, among other structures, 

 to the alimentary canal. From the time that the outer surface of the ball is 

 depressed at one point, until the hollow space is cut off from the outer surface, 

 is the gastrala stage. 



1 Driesch, H., ' Uber Seeigelbastarde, " Archiv fur Entwicklunysmechanil; 

 Bd. 16, 1903. 



2 Boveri, Th., "Uber den Einfluss der Samenzelle auf die Larvcucharaktere 

 der Echinideu," Archiv fur EntwicUunysmechanik, Bd. 16, 1903. 



3 Loeb, J., " Uber die Befmchtung von Seeigeleiern durch Seestemsamen," 

 Archiv fiir der gesammte Physiologic, Bd. 99, 1903. 



4 Fischer, A., " Entwicklung und Organdifferenziemng, " Archiv fiir Enhcick- 

 lungstnechanik, Bd. 15, 1903. 



5 Wilson, E. B., "Experimental Studies on Germinal Localisation 

 (1) The Germ-Regions in the Eggs of Dentalium ; (2) Experiments on the 

 Cleavage-Mosaic in Patella and Dentalium," Journal of Experimental Zoology, 

 vol. i., 1904. 



