MUSEUM OF COMPARATIVE ZOOLOGY. 495 



into) the embryo-sac. The author thinks this assumption is supported 

 by certain results obtained by Maxime Cornu (Comptes rendus de I'Acad. 

 des Sciences, Paris, Tom. LXXXIV. p» 134). Of the two nuclei which 

 are found in the tip of the pollen tube at the beginning of fertilization, 

 the one in the rear (from the "vegetative" cell) is the first to disappear. 

 For this reason Strasburger says it may be that it is the substance of the 

 other nucleus which is more especially concerned in the act of fecundation 

 (p. 487), its substance being preserved in a nuclear form up to the very 

 instant of fecundation. The homogeneous condition here brought about 

 just before fecundation by the disappearance of every nuclear structure 

 makes the fecundation of phanerogams in a sense parallel with that of 

 the higher cryptogams where the spermatozoid, though a "formed" 

 structure, has no nuclear differentiation, but is a homogeneous band in 

 which the nuclear substance is probably distributed uniformly. In 

 plants where the copulating elements remain in an indifferent condition, 

 i. e. indistinguishable from each other, there may be more than two 

 such individual elements concerned in the act (e. g. Spirogyra, etc.). 

 With a differentiation of the sexual products the possibility of this ap- 

 pears to cease.* Why in general only one spermatozoid is admitted to 

 an egg, remains undecided. In the case of plants it may be owing to 

 the extremely rapid production of a cellulose envelope, yet molecular 

 processes of an altogether different character may in this case come into 

 action. 



The experiments of Fol and 0. Hertwig ('78) in the fecundation of 

 starfish eggs are in many ways mutually confirmatory. In others their 

 opinions differ. According to Hertwig eggs that are fertilized any time 

 between the formation of the first maturation spindle and the completion 

 of the e,^^ nucleus afford evidence of the penetration of only a single 

 spermatozoon ; but there is this difference in the two cases. When the 

 fertilization takes place at the earlier date, the male and female pronuclei 

 attain equal size before their confluence, although the male pronucleus 

 remains comparatively small and with little influence on the surround- 

 ing protoplasm up to the time the second polar globule is formed. On 

 the other hand, when fertilization ensues only after the formation of the 

 egg nucleus, the male aster grows rapidly in size, but the male pronucleus 

 remains a much smaller structure than the female. Hertwig explains 

 this by supposing that in the latter case the female pronucleus has be- 



* I do not understand how the observations of the author necessarily exclude the 

 nucleus of the "vegetative" pollen-cell from participation in the act of fecundation ; 

 and if not excluded, the above sentence does not seem entirely justified. 



