1902.] NATURAL SCIENCES OF PHILADELPHIA. 



755 



ment of the gonad and the liberation of the eggs or spermatozoa 

 imbedded in its substance. The contractions of the umbrella in 

 swimming put a strain upon the subumbral walls and help to rup- 

 ture the epithelium of the gonads. What the cytological change is 

 which precedes the extrusion of the sexual elements or the nature of 

 the causes which effect this change are matters of uncertainty. 

 We know only that these conditions can be artificially induced by 

 means to be mentioned presently. The process of dehiscence occu- 

 pies only a very few moments. Most of the eggs Avhich are ready 

 for fertilization are extruded all at once, coming out of the bell- 

 cavity in a cloud at each contraction of the marginal ring. Two 

 or three minutes after dehiscence begins only a few belated eggs 

 remain to be loosened from the gonads and expelled from the 

 subumbral cavity one at a time. PI. XXXI, fig. 5, is drawn 

 from a sketch of a medusa in the act of spaw'ning. The specimen 

 was held inverted under the microscope in a watch-glass. 

 Although not free to swim it went through the motions, contract- 

 ing the bell rhythmically. In this way the softened ectodermal 

 tissue of the gonads was ruptured and the eggs expelled. Little 

 round pits are left by the eggs, like bullet-molds. 



The earliest date at which fertile medusse have been found was 

 the first of July; the latest, the last -sveek of September, The 

 period of maximum sexual activity is from the middle of July to 

 the middle of August. 



B. Periodicity.— As stated by Murbach (1895), the eggs of 

 Gonionema are deposited with great regularity. During the earlier 

 part of the summer dehiscence takes place at about 8 P.IM., but 

 later in the season, when dusk comes earlier, the medusie spawn 

 at 7 or even as early as a quarter past 6. Extrusion of the eggs 

 may be artificially induced, In this respect Gonionema differs 

 markedly from some other marine animals which exhibit equal 

 definiteness in the spawning time. Dr. jNIurbach found that after 

 the medusfe had been shut up in a dark place for an hour, even dur- 

 ing the daytime, they would deposit eggs and sperm. My experi- 

 ments show that this is more likely to be the case in the afternoon 

 than earlier in the day; before 2 o'clock in the afternoon, an hour 

 in the dark would sometimes bring about a deposition of a small 

 number of eggs, and if the period was lengthened to an hour and 

 a half, a slightly larger number of eggs were found in the water. 



