222 bulletin: museum of comparative zoology. 



To recapitulate. In the impregnated egg of Ciona two archoplasmic 

 masses can be recognized, one in connection with each of the pronuclei. 

 That derived from the spermatazoon. is much the more energetic of the two, 

 and is alone concerned in bringing the pronuclei together. While the pro- 

 nuclei are still a considerable distance apart, the male archoplasm divides 

 into two distinct attraction spheres, between which the first cleavage spindle 

 later forms. Tlie female archoplasm degenerates, taking no part whatever 

 in the formation of the first cleavage spindle. There is accordingly in the 

 fertilization of Ciona no union of male and. female archoplasms. 



Let us compare briefly these conclusions with those of other recent 

 observers on the subject of the attraction sphere in fertilization. 



The fertilization of the Tunicate egg has been studied hitherto by 

 Boveri ('90) aud Julin ('93). Boveri's observations, made on Ciona 

 intestinalis and Ascidia mentula, were, as he states, incomplete on 

 account of an accident to his preserved material. It was his opinion 

 that no astral radiations (" Polstrahlungen ") are present in the matura- 

 tion of the egg, and that the two asters of the first cleavage spindle are 

 derived by division from a single one arising in connection with the 

 spermatazoon soon after its entrance into the egg. Julin was able to 

 confirm on Styelopsis grossularia the observations of Boveri, and to sup- 

 plement them, as he says, by demonstrating at the centre of each aster 

 of the first cleavage spindle a centrosome. No figures, however, accom- 

 pany Julin's paper ; moreover, he states that his observations were 

 restricted to two stages, corresponding to those shown in Boveri's 

 Tafel XII. Figs. 27 and 29. 



Though my own conclusions are in entire agreement with those of 

 Boveri and Julin as to the derivation of the attraction spheres of the 

 first cleavage spindle exclusively from the spermatazoon, my observations 

 differ from theirs regarding certain minor points, as the reader may 

 learn by consulting the papers cited. 



On the subject of fertilization in groups of animals other than the 

 Tunicata there is an enormous literature. I shall refer to only a few of 

 the most recent papers. 



In 1891 Fol described the famous "quadrille of the centres" as occur- 

 ring in the fertilization of the sea-urchin egg. According to his account, 

 there arises in the egg from the tip of the spermatazoon, a centre of 

 attraction (" spermocentre "), which later divides. In connection with 

 the egg nucleus appears another centre of attraction ("ovocentre"), 

 which likewise divides. Upon the meeting of the pronuclei, each half- 

 spermocentre unites with a half-ovocentre to form an astrocentre. The 



