TYPE OF THE PHYLACTOLAEMATOUS LARVA. 



35 



We shall have to call the external cell-layer of the embryo the ectoderm, but 

 it must not be forgotten that this layer contains indifferent embryonic material 

 from which will be produced the future rudiment of the polypide, including the 

 ontodermal organs. 



The next ontogenetic phenomenon is the development of the 

 rudiment of the first polypide. The larvae of some fresh -water 

 Bryozoa (Plumatella) at the free - swimming stage contain two 

 well - developed polypides (Fig. 19 A), one of which, however, 

 always seems to develop as the older primary polypide, and the 

 other as a precocious daughter bud. The larva of Plumatella 

 Jruticosa, on the contrary, contains, according to Allman, only one 

 primary polypide, and in Cristatella also, the second polypide does 

 not seem to 

 develop so early 

 as in the above- 

 named form. 

 The polypides 

 (Fig. 17, p) arise 

 as simple invagi- 

 nations of the 

 bilaminar wall 

 of the embryo. 

 Their further 

 development will 

 be described 

 more in detail 

 below (p. 37). 

 The first indi- 

 cations of the 

 polypide - invagi- 

 nation are found 

 in the form of 

 a simple thicken- 

 ing of the body- 

 wall. Daven- 

 port states that, 



in Plumatella, the rudiment of the second polypide appears from 

 tin- first independent of the primary polypide. 



The conditions in Cristatella are, according to Davenport, somewhat 

 diiierent. We shall see below (p. 47) that in this form every new bud is 

 intimately connected at its commencement with an older bud, and that the 



Fig. 17.— Two later ontogenetic stages of the embryo ot Plumatella 

 (after Kraepelin). ec, ectoderm of the embryo ; /, mantle-fold ; 

 m, mesodermal layer ; o, ooecium ; p, polypide-rudiment ; pi, 

 placenta. 



