ON THE EMBRYOLOGY OF THE ECTOPROCTA. 3 



appears to be characteristic of all the Ctenostomata and Cheilosto- 

 inata, as described by Repiachoff (14), Barrois (1,2) and others. 

 The segmentation-cavity was first distinguished in embryos com- 

 posed of sixteen cells, which are disposed in four longitudinal 

 rows of four each, two rows belonging to the oral half of the embryo, 

 and two to its aboral half. At the 48-cell stage, the aboral half is 

 composed of thirty-two cells, and the oral half of twelve cells, whilst 

 four cells are internal. In the aboral region, the arrangement is as 

 follows : 



(1) two longitudinal rows, of four cells each, disposed sym- 

 metrically, right and left of the median plane, and occupying the 

 centre of the aboral surface, 



(2) a ring of eight cells completely surrounding the central 

 group of cells, and in its turn surrounded by 



(3) a peripheral ring of sixteen cells, which are, as Barrois has 

 shown, the commencement of the ciliated ring. 



The oral half consists of a central group of four large cells, 

 surrounded by twelve peripheral cells. 



The segmentation-cavity is at this stage fairly large, but is 

 partially filled by four cells which are situated immediately above 

 the central oral cells, and which have probably been derived from 

 these latter. The four cells lying in the segmentation-cavity arc 

 the commencement of the hypoblast. 



At a somewhat later stage I have observed the existence of a 

 wide depression, the blastopore, situated in the middle of the oral 

 surface and continuous with a somewhat irregular cavity surrounded 

 by several large hypoblast cells. 



At a subsequent stage, when the segmentation-cavity is 

 partially filled by a large mass of cells (probably representing 

 hypoblast jdus nipsoblast), the blastopore appears to have closed, 

 whilst still later the segmentation-cavity is completely obliterated 

 by the internal cell-mass, and the various organs of the larva are 

 commencing to make their appearance. 



In the first stage selected for figuring (PI. I, fig. 1, a median 

 longitudinal section through a young embryo) most of these organs 

 are already partially established. The yolk-spherules are still to 

 be found in all the tissues of the embryo. Two of the large cells 



1—2 



