238 RECOKD OF CURRENT RESEARCHES RELATING TO 



faces alternate in two ways, that is, the faces arc on all four sides of 

 the almost cylindrical branches, and they alternate in each of the 

 opposing cells arising on the side of the centre of its alternate 

 neighbour. Tetraplaria (australis) is the name proposed. 



Embryology of Bowerbankia.* — Hcrr Kepiachoff follows up his 

 previous important papers f on the development of the Polyzoa by now 

 showing the agreement which subsists between the earlier stages of 

 BowerhanMa (two species) and the Chilostomatous Tendra and Lepralia 

 pallasiana. 



The polar corpuscle marks the future posterior side ; after 

 its exit the cleavage of the egg-cell begins; the third cleavage 

 divides the embryo into a dorsal and a ventral portion, enclosing a 

 cleavage cavity. After two stages like the corresponding ones in 

 Tendra, the smaller dorsal cells divide, and then the four median 

 ventral cells are pushed into the cavity and form the endoderm. 

 After the closing uj) of the ectoderm it acquires a thickening on the 

 dorsal side above mentioned ; the true position of this thickening with 

 regard to the " primitive mouth," and therefore to the body of the 

 larva, was difficult to decide, and this interpretation of the relations 

 of the parts leads to the unexpected conclusion that this •' mouth " is 

 on the opposite side of tho body in BowerhanMa to that which it 

 occupies in the Chilostomatous larva, and that the thickening does not 

 represent in position the sucker of the Chilostomata. 



The second special feature of the larva is a zone of cilia, carried 

 by a single row of very large cells which present a fusiform aspect 

 ■when viewed from above the embryo. Of the two spaces at the sides 

 of the zone, the larger is rather thin-walled, the smaller is remarkably 

 thick at its centre — which constitutes the dorsal thickening above- 

 mentioned — but becomes thinner towards the zone ; at a later stage 

 these parts become less sharply defined from their surroundings, the 

 ciliary zone takes an oblique position with regard to the long axis of 

 the embryo, the thickening becomes extended in the direction of that 

 axis, and now consists of two or three successive divisions. At later 

 stages this extension is continued, and the ciliary zone takes the form 

 of two practically parallel rows of cells, connected with each other 

 above and below, and lying chiefly right and left of the thickening ; 

 they take a turn round the ventral side of the anterior end of the 

 embryo, and here form a border to the anterior end of the thickening. 



At this stage the mantle is indicated by a ventral invagination at 

 the hinder end of the body. The cells (probably all) of the ciliary 

 zone now divide so as to create two parallel rows in it ; they also 

 appear to divide longitudinally, and the zone, or most of it, becomes 

 now indistinguishably fused with the thickening, forming the structure 

 called by the author elsewhere the " ventral ring." The mantle now 

 assumes its special characters. The endoderm — whose characters 

 differ somewhat in different individuals of the same stage, consisting 

 sometimes of a single layer of cells enclosing other cells scattered 

 or aggregated, sometimes of an apparently compact mass of cells 



* ' Zool. Anzeiger,' ii. (1879) p. GGO. f Ibid., i. (1878) No. 10., &c. 



