DEVELOPMENT OF FLUSTRELLA HISPIDA. 451 



Fertilisation, and the Passage op the Egg into the 

 Tentacle Sheath. 



Fertilisation. — The act of fertilisation has not yet been 

 observed, only such ova as were either preparing for, or had 

 ah-eady undergone, fertilisation having been obtained. It has 

 already been stated that the spermatozoa and ova ripen for 

 the most part at different times, and no ripe spermatozoa 

 have been observed in the zooecial cavity after March. 



Passage of the Eggs into the Tentacle Sheath. — 

 The development of the ova takes place within the tentacle 

 sheath, and the eggs pass from the zooecial cavity into the 

 tentacle sheath in the interval between the degeneration of a 

 polypi de and the formation of a new bud. This bud attains 

 complete maturity, and not, as has been described by Joliot 

 (14) in the case of Valkeria cuscuta, only partial deve- 

 lopment. As has been mentioned, from four to five eggs 

 generally ripen at the same time, and these enter the tentacle 

 sheath together and develop side by side. As development 

 proceeds, the larvse, while still enclosed in the tentacle sheath, 

 increase in size, and gradually come to fill the entire cavity 

 at first occupied by the polypide, and the latter now ceases 

 to exist. Those zooccia occupied by advanced larva) contain 

 large quantities of a slimy, mucus-like substance, which 

 suri'ounds the developing embryos. 



Segmentation, and the Formation of the Germinal Layers. 



The earlier stages of segmentation have been studied in 

 detail in the hope of elucidating the problem of the origiu 

 and subsequent history of the mesoderm and of the endoderm 

 in this form. So far, however, the formation of mesoderm 

 has not been definitely traced, but it is hoped that it may 

 be possible to determine this point more satisfactorily at a 

 later date. 



The Primitive Cleavages. — After fertilisation the egg 

 becomes separated from the vitelline membrane by a wide 

 space (PI. 23, fig. 37). The first cleavage (Pi. 23, figs. 37-42) 



