MATURATION OF PARTHEAOGENETIC EGGS 28 1 



theoretical meaning of maturation, the suggestion is made that 

 parthenogenesis may be due to failure on the part of the Q.gg 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 afterward maintained by 

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

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

 discovered, however, that the parthenogenetic eggs of PolypJicmns 

 (one of the Daphnidae) produce a single polar body. This observa- 

 tion was quickly followed by the still more significant discovery by 

 Blochmann i^'^'i) that in Aphis tJie partlicnogenetic eggs produce a single 

 polar body, zvJiile the fertilized eggs produce tiuo. Weismann was able 

 to determine the same fact in ostracodes and Rotifera, and was thus 

 led to the view ^ which later researches have entirely confirmed, that 

 it is the second polar body that is of special significance in partheno- 

 genesis. 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 {'Sy, i) dis- 

 covered that in Ascaris the second polar body might in exceptional 

 cases remain in the egg and there give rise to a resting-nucleus indis- 

 tinguishable 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 : ''Parthenogenesis is the result of fertilisation 

 by the second polar body.'' ^ 



This conclusion received a brilliant confirmation through the obser- 

 vations of Brauer ('93) on the parthenogenetic Qgg of Arteviia, 

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

 Blochmann ('88-89) ^^^ found that in the parthenogenetic eggs 

 of the honey-bee tiuo polar bodies are formed, and Platner discov- 

 ered the same fact in the butterfly Liparis ('89) — a fact which 

 seemed to contradict Boveri's hypothesis. Brauer's beautiful re- 

 searches resolved the contradiction by showing that there are two 

 types oi parthenogenesis which may occur in the same animal. In the 

 one case Boveri's conception is exactly realized, while the other is 

 easily brought into relation with it. 



{a) In both modes typical tetrads' are formed in the germ-nucleus 

 to the number of eighty-four. In the first and more frequent case 



i '83, p. 622. 2 Essay VI., p. 359. ^ I.e., p. 73. 



