202 REDUCTION OF THE CHROMOSOMES 



G. MATURATION OF PARTHENOGENETIC EGGS 



The maturation of eggs that develop without fertilization is a sub- 

 ject of special interest, partly because of its bearing on the general 

 theory of fertilization, partly because it is here, as I believe, that one 

 of the strongest supports is found for the hypothesis of the individ- 

 uality of chromosomes. In an early article by Minot ('77) on the 

 theoretical meaning of maturation the suggestion is made that 

 parthenogenesis may be due to failure on the part of the egg to 

 form the polar bodies, the egg-nucleus thus remaining hermaphrodite, 

 and hence capable of development without fertilization. This sug- 

 gestion forms the germ of all later theories of parthenogenesis. Bal- 

 four ('80) suggested that the function of forming polar cells has been 

 acquired by the ovum for the express purpose of preventing parthe- 

 nogenesis, and a nearly similar view was afterwards maintained by 

 Van Beneden. 1 These authors assumed accordingly that in par- 

 thenogenetic eggs no polar bodies are formed. Weismann ('86) 

 soon discovered, however, that the parthenogenetic eggs of Poly- 

 phemus (one of the Daphnidse) produce a single polar-body. This 

 observation was quickly followed by the still more significant dis- 

 covery by Blochmann ('88) that in Aphis the parthenogenetic eggs 

 produce a single polar body while the fertilized eggs produce two. 

 Weismann was able to determine the same fact in ostracodes and 

 rotif era, and was thus led to the view 2 which later researches have 

 entirely confirmed, that it is the second polar body that is of special 

 significance in parthenogenesis. Blochmann observed that in insects 

 the polar bodies were not actually thrown out of the egg, but 

 remained embedded in its substance near the periphery. At the 

 same time Boveri ('87, i) discovered that in Ascaris the second polar 

 body might in exceptional cases remain in the egg and there give 

 rise to a resting-nucleus indistinguishable from the egg-nucleus or 

 sperm-nucleus. He was thus led to the interesting suggestion that 

 parthenogenesis might be due to the retention of the second polar 

 body in the egg and its union with the egg-nucleus. " The second 

 polar body would thus, in a certain sense, assume the role of the 

 spermatozoon, and it might not without reason be said : Partheno- 

 genesis is the result of fertilization by the second polar body." ^ 



This conclusion received a brilliant confirmation through the obser- 

 vations of Brauer ('93) on the parthenogenetic egg of Artemia, 

 though it appeared that Boveri arrived at only a part of the truth. 

 Blochmann ('88-89) na d found that in the parthenogenetic eggs of 

 the honey-bee, two polar-bodies are formed, and Platncr discovered the 



1 '83. p. 622. 2 Essay VI., p. 359. 3 I.e., p. 73. 



