CASTLE : EMBRYOLOGY OF CIONA INTESTINALIS. 223 



two astroceutres arise on opposite sides of the cleavage nucleus, and 

 between them the first cleavage spindle forms. 



A short time after the publication of Fol's paper, Guignard ('91) 

 described as occurring in the fertilization of a flowering plant a similar 

 union of male and female centres of attraction (" spheres directrices "). 

 More recently Conklin ('94) has observed its occurrence in the case of a 

 moUusk, Crepidula. 



Fol's observations, however, are flatly contradicted by the careful 

 studies of Wilson and Mathews ('95) on three diff'erent genera of Echi- 

 noderms. They find that "the central archoplasm sphere ('attraction 

 sphere ') of the cleavage amphiaster is derived by direct and unbroken 

 descent from the central mass of the sperm-aster without visible partici- 

 pation of an egg-aster." 



Fick ('93) also observed that in the fertilization of a Vertebrate, Axo- 

 lotyl, the centrosomes of the first cleavage spindle are derived exclu- 

 sively from the spermatozoon. Brauer ('92) arrived at a similar 

 conclusion regarding the fertilization of a crustacean, Branchipus, and 

 Mead ('95) regarding a worm, Chaetopteris. These observations are in 

 entire agreement with those made prior to Fol's announcement of the 

 " Quadrille " by Boveri ('88) on Ascaris and Sagitta, and by Vejdovsky 

 ('88) on Rhynchelmis. Boehm ('88) had also expressed with some 

 caution a similar view regarding Petromyzon. 



On the other hand, Wheeler ('95), in a paper published simultane- 

 ously both with that of Wilson and Mathews and with that of Mead, 

 states that in Myzostoma both centres of attraction arise in connection 

 with the egg nucleus, none whatever being produced by the spermatozoon. 



Summary on Maturation and Fertilization. 



(1) In a majority of the animals in which fertilization has been most 

 recently studied the attraction centres of the first cleavage spindle are de- 

 rived from the spermatozoon and from the spei-matozoon only. 



(2) But in the fertilization of at least one animal., and undouhtedly in 

 all cases of parthenogenetic development.^ the attraction centres arise solely 

 in connection with the egg nucleus. • 



(3) Both these facts prove conclusively that the archoplasm, or " organ 

 of division," is not a hearer of heredity, since in fertilization it may be 

 derived from the sexual product of one parent only, whereas it is a well 

 recognized law that heritable substance is contributed to the offspring by 

 both parents equally. 



(4) If the archoplasm is furnished in some cases by the sperm only 



