16 



BRYOZOA ECTOPROCTA. 



angles to the first ; this stage, in consequence of an equatorial furrow, 

 passes over into the regular eight-celled stage (Fig. 6 ^1). A small 

 blastocoele can frequently be observed as early as this stage, and the 

 animal pole can be distinguished from. the vegetative pole by a differ- 

 ence of size — often only very slight — in their respective blastomeres. 

 In the further segmentation there is a peculiarity worthy of note. 

 The sixteen-celled stage is reached through the appearance of two 



c 



Fig. 6.— Cleavage and embryonic stages of Bugula calathus (after Vigelius). A, stage with 

 eight blastomeres. B, stage with sixteen blastomeres. C, stage with thirty-two blasto- 

 meres. All seen from above. The dotted lines in A and B indicate the direction of the 

 next cleavage planes. D, blastula-stage in vertical section. E and F, gastrnla-stages. 

 G, a later ontogenetic stage, c, corona-cells ; ec, ectoderm ; en, entoderm ; /, cleavage- 

 cavity ; z, central tissue derived from the primary entoderm. 



furrows lying on either side of the first meridional plane and running 

 parallel to it. An embryo thus results, each half of which consists 

 of eight cells (in two rows of four, Fig. 6 B). A similar division 

 leads to the thirty-two-celled stage (Fig. 6 G), two furrows forming 

 parallel to and at the sides of the second meridional plane. Each 

 half of tins embryo consists of four rows, each containing four cells. 

 This stage is followed by less regular divisions and results in a 



