52 Cuming s — Development of some Paleozoic Bryozoa. 



I. Classification of the Stages of Growth. 

 Embryonic Stages. 



The protembryo includes the ovum and its segmented stages 

 up to the formation of the blastula. Cleavage is total and 

 almost equal (Korschelt and Heider 17 ). The animal and vege- 

 tative poles can be distinguished by a slight difference in the 

 size of their respective blastomeres. 



The mesembryo (blastula) (fig. 1) is of lenticular shape, with 

 a fair-sized cleavage cavity. 



The metembryo (gastrula) (figs. 2, 3) is formed by the ingres- 

 sion of four endoderm cells. The coronal cells are defined at 

 this stage (c, fig. 2) and later become Very conspicuous. The 

 four endoderm cells multiply so as to completely fill the 

 blastocoele, thus forming a planula. 



The neoembryo (figs. 4-8 and 17-25) includes the stages from 

 the formation of the planula to the completed free-swimming 

 larva (fig. 8). During these stages the endodermal cells multiply, 

 according to Barrois 2 differentiating into endoderm and meso- 



17 



16 



Figures 16-19. — Early embryonic stages of a Cyclostomatous Bryozoan, 

 Phalangella ( = Tubulipora). (After Barrois.) 



16. Metembryo (pseudoblastula) ; 17, 18, 19. (Pseudogastrula) invagina- 

 tion of the adhesive organ ; stages corresponding to figs. 6 and 7. 



or, oral region ; C, invagination cavity. 



Figure 16 x 300 ; figures 17-19 x 160. 



derm* (figs.4, 5). The cells of the corona increase in size and 

 become covered with cilia ; a disclike thickening bearing stiff 

 setse, the retractile disc, develops in the aboral half. This is 

 encircled by a depression, — the mantle cavity. An invagination 

 develops in the posterior portion of the aboral half, to form the 

 sucker, or adhesive organ (ap). An alimentary canal may 

 develop at this stage in some types of larvae (see figs. 31-35 

 and 22, 25). 



The typembryo (figs. 9, 10) is formed by the evagination of 

 the adhesive organ and reversal of the mantle (fig. 10). 

 This gives rise to the first sedentary stage, which is passed 

 through with great rapidity, and is immediately succeeded by a 

 complicated metamorphosis involving the degeneration of most 

 of the larval organs. 



* This has not been observed by other workers. 



