90 



INVEETEBEATA 



CHAP. 



pole and slowly make their way through the yolky portion of the 

 egg. The four segments then divide into eight, not as one finds in 



B 



W?i 



nic 



irnc 



mic 



Fic. 70. — Two views of the developing egg of Bcroe ovata seen froin aliove. 

 (After Ziegler.) 



A, the first-formed micromeres have just divided and a second micromere has been budded off from 

 each macromere. B, the daughters of the first micromeres have divided again, mid^, daughters of 

 first-formed micromeres ; mic2, micromeres budded off subsequently. 



other eggs by a circular furrow, but by oblique almost vertical furrows, 



which separate off four inner larger cells from four outer smaller cells. 



The eight cells are arranged in two linear rows of four each. 



These rows stand opposite to one another and form an ellipse-like 



A B 



mac 



Fig. 71. — Oral and aboral views of the emljryo of Beroe ovata iu a later stage. 

 (After Ziegler. ) 



A,' from aboral pole. B, from oral pole, trutc, raacromeres ; r, small ectoderm cells which later 

 will develop into the ribs of the adnlt. 



figure, the long axis of which is at right angles to the stomach plane 

 of the adult, and is identical with the funnel plane of the adult 

 Ctenophore. These eight cells are termed the macromeres. Then each 

 macromere, by unequal division, buds off a much smaller cell termed a 

 micromere at the upper pole, and thus a 16-cell stage is reached. 

 Now Ziegler (1898) has shown that the process of division of a 



