BEAEERS OF HEREDITARY UNITS 213 



This evidence failed, however, in so far as there might 

 be present a certain amount of nuclear plasm in the sperm- 

 head that is carried in with the head, and if so, would be 

 later mixed with the egg cytoplasm. The discovery that 

 at the base of the sperm-head there is present in some eggs 

 a centrosome that becomes, through division, the dynamic 

 centre of the next division, opened the door to suspicion 

 that the sperm might bring in other things than the chro- 

 mosomes to influence development, and hence heredity. 



In conclusion then, while it may be said that the evi- 

 dence that the sperm-head alone enters the egg may be 

 claimed as favorable for the chromosome view, it cannot 

 be accepted as critical proof, because it is uncertain 

 whether other things also may not be brought in besides 

 the chromatin of the sperm. 



Boveri's evidence for chromosomal heredity from di- 

 spermic sea urchin eggs was open to less objection. It was 

 known that when two sperms enter the sea urchin's egg 

 simultaneously, the first division of the egg is into three 

 or into four parts, because four (instead of two) division- 

 centres appear in these dispermic eggs. It was also known 

 that these eggs rarely produce normal embryos or larvae. 

 Boveri, studying the mode of division of the dispermic 

 eggs, found that there was an irreg-ular distribution of 

 the chromosomes to the three or four poles that appear, 

 and consequently to the three or four resulting cells (Fig. 

 98). The abnormal development of the whole egg that 

 generally follows might be ascribed to the irregular dis- 

 tribution of chromosomes to different regions ; for, quite 

 apart from the specific nature of each chromosome or 

 group of chromosomes, the activity of one region being 

 quantitatively different from that of a corresponding 

 region in another part of the egg might be responsible for 

 the failure to develop normally. But Boveri went further 

 in his analysis. He shook apart the three or four blasto- 

 meres coming from dispermic eggs (by using Herbst's 

 calcium-free sea-water method), and compared the num- 



