378 bulletin: museum of comparative zoology. 



5. Fertilization. 



Both methods of union of the germ nuclei — fusion and mere apposi- 

 tion — have already heen observed among coelenterates. The first has 

 been described in greatest detail by Boveri ('90). In Tiara, according 

 to this author, the sperm nucleus is a small, compact chromatic mass, 

 surrounded by a clear area ; a condition much like the one seen in Gon- 

 ionemus at a corresponding period (p. 354). This clear area then be- 

 comes continuous with the " vacuolar space " of the egg nucleus, and 

 both maternal and paternal chromatin become enclosed by a single 

 membrane. Boveri furthermore made the extremely important discovery 

 that the maternal chromosomes are formed before the paternal chromatin 

 mass undergoes any changes, thereby corroborating for coelenterates the 

 earlier results of Mark ('81), and especially van Beneden ('83), that 

 the chromosomes of the first cleavage spindle are purely maternal or 

 paternal. 



A similar inclusion of the paternal chromatin mass by the egg nu- 

 cleus occurs in Gonothyrea (Wulfert, : 02) and, as i have shown, it is 

 the visual mode of union of the germ nuclei in Gonionemus. In other 

 coelenterates in which fusion takes place — e. g., Hydra (Brauer, '9l a ) ; 

 Tubularia (Brauer, '9l b ) ; Clava (Harm; :02), and Cordylophora (Mor- 

 genstein, :0l) — the two germ nuclei are of about equal size at the 

 time of their union. Fusion is not, however, invariable, for mere appo- 

 sition of the germ nuclei occurs in Aequorea (Hacker, '92 a ), as well as, 

 exceptionally, in Gonionemus. 



The occurrence of two alternative methods of nuclear union in one 

 species, such as is seen in Gonionemus, although uncommon, is not unpar- 

 alleled. So long ago as 1887 Boveri ('87) described a similar condition 

 in Ascaris, finding that fusion occasionally replaces the more usual appo- 

 sition ; and more recently Wulfert (:02) has described a similar state of 

 affairs in Cordylophora. E. B. Wilson (: 00) has shown that there is no 

 essential difference between the two methods of nuclear union ; the oc- 

 currence of one or the other depending on the time which elapses between 

 the penetration of the spermatozoon and the meeting of the nuclei. 

 " On general grounds we may confidently maintain that the distinction 

 between the two ... is due to corresponding differences in the rate of 

 development of the germ nuclei, or in the time that elapses before their 

 union " (E. B. Wilson, : 00, p. 205). It is, I believe, the first of these two 

 factors that determines the mode of union in Gonionemus, fusion taking 

 place when the sperm nucleus is small at the time of union, apposition 

 when it is of nearly the size of the egg nucleus. 



