STRTCTURE AND DEVKLUPMEN T OE COELOl'LANA. 47 



first polar body. In addition, there is to be seen occasionally a low 

 prominence somewhere on the surface of the egg-body, but generally 

 in the neighbourhood of the polar bodies {ent. <:); this apparently repre- 

 sents the entrance-cone of the spermatozoon. 



Segmentation . 



The segmentation of the &o^ advances essentially in the same way 

 as in ordinary ctenophores, known to us through Ziegler's work on 

 Bcroe ('98) and Yatsu's on Beroe and Callianira ('ii). The first cleavage 

 (fig. 2) is meridional. The cleavage furrow appears at the vegetative 

 pole in the neighbourhood of the polar bodies and advances towards the 

 opposite animal pole. About the cleavage head (fig. 3), one can 

 recognize a clear radial arrangement of the ectoplasmic granules and 

 some delicate fibrous bodies projecting from the surface of the &^^- 

 The second cleavage plane (fig. 5) is also meridional, but perpendicular 

 to the first; it divides each cell into two equal daughter cells (fig. 6). 

 As in ordinary ctenophores, the first cleavage plane corresponds with 

 the pharyngeal or sagittal, and the second with the tentacular or 

 transverse plane of the future larval as well as adult body. The next 

 or third cleavage plane is "diagonal", as in the eggs of ordinary 

 ctenophores (fig. 7). It traverses each cell obliquely, beginning at a 

 point between the vegetative pole and the equator of the egg-body on 

 the outer side and ending between the animal pole and the equator on 

 the inner side; it cuts each cell into two daughter cells of somewhat 

 different size, a larger cell placed nearer the vegetative pole and a 

 smaller nearer the animal pole. The four smaller cells are situated 

 in two groups each consisting of two cells situated in close contact with 

 each other, one on either side of the second cleavage plane (fig. 9). As 

 is well-known, such an arrangement of the blastomeres in the eight-cell 

 stage is characteristic of the cleavage in the Ctenophora. For the sake 

 of description, I shall name after Yatsu the four larger cells of this 

 stage M (middle-cells) and the four smaller cells E (end-cells). In the 

 fourth cleavage, each of the eight cells gives rise to a very small cell 

 at the animal pole and thus the i6-cell stage is attained (fig. lo). This 

 .stage consists of eight large cells (macromeres) and eight very small 

 cells (micromeres), of which the former are situated nearer the vegetative 

 pole and the latter nearer the animal pole, so that we ma}' call those 

 two poles the macromere and micromere pole respectively. Of the eight 



