54 BRYOZOA ECTOPROCTA. 



of the formative by the cystigen half was concluded (Fig. 26 (7, p). 

 The germ-disc, in which must also be included the adjacent lower 

 mesodermal layer, now becomes invaginated into the yolk -mass. 

 The closing of the aperture of this invagination gives the polypide- 

 rudiment the form of a bilaminar sac, which develops further 

 according to the type described above (p. 37 and Fig. 27 B). The 

 only difference in this case is that the body-cavity and all its 

 derivatives (the lophophore- cavity, the circular canal, etc.) are 

 originally completely filled with food -yolk (d), which disappears 

 only gradually through the absorption that takes place during 

 further development. 



The rudiments of the future buds are early to be recognised as 

 epithelial thickenings (Fig. 27 J3, ks) at the margin of the stato- 

 blast (corresponding to the oral side of the primary polypide), from 

 which, by invagination, the second and then the third polypide- 

 rudiments are formed. These, from the time of their first develop- 

 ment, are thus independent of the germ-disc. In a similar way, 

 according to DAVENPORT, in the embryos of Plumatella, the rudiment 

 of the second polypide appears independently of the first. All the 

 buds that develop later, on the contrary, arise in connection with 

 an older polypide-rudiment, as was described above, for the type 

 of budding in which the polypide develops first. 



C. Winter Buds (Hibernacula). 



In the fresh -water Gymnolaemata, Victor ella and Paludicella, 

 statoblasts do not develop. In Paludicella, isolated individuals 

 (zooecia with rudimentary polypides) persist in an encysted con- 

 dition. These individuals, which are enclosed in strong, highly 

 calcified, chitinous envelopes (ectocysts), are known as winter buds 

 (hibernacula, VAN BENEDEN). In the spring the envelope bursts, 

 and there emerges from it an individual covered with a delicate 

 chitinous cuticle, which by budding gives rise to a new colony ; 

 the budding may occur while still in the hibernaculum. The winter 

 buds here represent merely the resting condition of the adult form. 

 Their development in the spring is essentially reducible to a process 

 of ecdysis. 



In a similar way, in Victorella, short stolons with closely crowded 

 knob-like rudiments of individuals persist through the winter, and 

 give rise in the spring to the first cylindrical cells of new colonies 

 (KRAEPELIN). 



