298 ECTOPROCTA. 



appearance of two vertical furrows, both parallel to one of the 

 original vertical furrows, so that the segmentation spheres at this 

 stage are arranged in two layers of eight each. In the next 

 stage segmentation takes place along two fresh vertical planes, 

 similar to those of the last stage, but at right angles to them, and 

 therefore parallel to the second of the two primitive vertical 

 furrows. At the close of this stage there are thirty-two cells 

 arranged in two layers of sixteen each, and when viewed from 

 the surface each of these layers presents a regularly symmetrical 

 pattern. Up to the stage with sixteen cells the two poles of the 

 egg, separated by the primitive equatorial plane of segmentation, 

 remain equal, but during the stage with thirty-two cells a 

 peculiar change takes place in the character of the cells at the 

 two poles. At the one pole, which will be spoken of as the oral 

 pole, the four central cells become much larger than the twelve 

 peripheral cells. 



The stages immediately following are still involved in much 

 obscurity, and have been described very differently by Barrois in 

 his original memoir (No. 298), and in a subsequent note (No. 

 307) \ In the latter he states that the four large cells of the 

 oral face become enclosed by the division and growth of the 

 twelve peripheral cells. They are thus carried into the interior 

 of the ovum ; and there divide into a central vitelline mass the 

 hypoblast and a peripheral mesoblastic layer. 



The eight peripheral cells of the aboral pole divide vertically, 

 and, owing to the eight central cells at the aboral pole dividing 

 transversely so as to form a protuberance on the aboral surface, 

 they constitute a transverse ring of large cells round the ovum, 

 which become ciliated and constitute the main ciliated band of 

 the embryo, corresponding to the ciliated band at the edge of 

 the vestibule of the entoproctous larvae. They divide the embryo 

 into an aboral and an oral region. The central part of the 

 aboral projection forms a structure which I shall speak of as the 

 ciliated disc. It probably corresponds with the ciliated disc in 

 the Entoprocta. An invagination is next formed on the oral 



1 The note (No. 307) refers in the first instance to the changes in the larvae of the 

 Chilostomata, but the similarity of the larvae of the Ctenostomata to those of the 

 Chilostomata renders it practically certain that the corrections, in so far as they apply 

 to the one group, apply also to the other. 



