32 GENETICS 



been furnished by certain critical experiments upon 

 the eggs of sea-urchins. Boveri found that he was 

 able in some instances to shake out the nuclei bodily, 

 chromosomes and all, from the mature eggs of the 

 sea-urchin, Sphcer echinus, and when there was added 

 in sea water to such enucleated eggs the sperm-cells 

 of an entirely different genus of sea-urchin, namely, 

 Echinus, the Echinus sperm-cells entered the Sphcer- 

 echinus eggs, which had been robbed of their nuclei, 

 and from this peculiar combination larvae developed 

 which exhibited only Echinus characters! 



Such cumulative circumstantial evidence as the 



(foregoing has convinced many that in the chromo- 

 somes we have visibly before us the carriers of 

 heredity. 



Several biologists, however, raise an objecting 

 voice to this theory, protesting against the mo- 

 nopoly of the heritage by the chromosomes. They 

 point out that there always exists an intimate 

 physiological relationship between the nucleus and 

 the cytoplasm, and that it is unreasonable to expect 

 the isolation of one from the other, since the two 

 must always act together as parts of an organic cell 

 unit. 



In sexual reproduction, moreover, some small 

 amount at least of spermatic cytoplasm in the form 

 of the so-called "middle piece," which is situated 

 between the head and the tail of the sperm-cell 

 (Fig. 15), may enter the egg about to be fertilized 

 along with the sperm " head " or nucleus, containing 

 the chromosomes. In this way the cytoplasm of the 



