204 



Mr. F. S. Conant on the Chcetognaths. 



out plainly in the living object at this stage is found in 

 sections to be the stalk-cell, which thus precedes the ovum in 

 its passage into the duct. The ovum itself now changes its 

 spherical shape hj sending out a projection, bearing the stalk- 

 cell, into the duct. Then by a progressive series of changes 

 in shape, such as is represented in fig. 2, each ovum makes 



OWD' 5 



4. 



Diagrams illustrating the successive stnges of constriction in the passage 

 of an egg into the oviduct. O, o\um; S, stalk-cell; IWU, inner 

 wall of the oviduct ; O WD, outer wall 



its way into the oviduct. It is evidently passing through 

 some such interspace in the wall of the oviduct, as was 

 supposed by Grassi. The process occupies about eight or ten 

 minutes. All the ova of one side pass through the successive 

 stages of constriction at essentially the same time, the anterior 

 being slightly in advance of the posterior. There may be a 

 difference of time, however, between the ova of the two sides. 

 The eggs remain in the oviduct from twenty minutes to 

 half an hour before being laid, during which time the layer 

 of gelatinous substance that surrounds each is thickened. 

 The chief change, however, is a gradual compression of the 

 eggs from before backward, so that they become ellipsoidal. 

 The process is aided by contractions of the ovary, especially 

 observed at the anterior end, and as a result the eggs are 

 pushed backward toward the external opening. When the 

 pressure has become great enough to overcome the resistance, 

 which seems to be offered largely by a kind of mucous plug 

 in the orifice, the eggs are extruded as two linear rows, one 

 from each oviduct, the whole comprising anywhere from a 

 few to sixty or seventy eggs. 



