158 OHAS. B. WILSON. 



The Escape from the Egg Membranes. — Cilia have 

 now developed over the entire external surface of the larva, 

 and as soon as it falls upon its side the rotary motion which 

 they produce presses the aboral surface against the membranes. 



This elongates the latter into an ellipsoidal form, and bends 

 the tuft of flagella at a sharp angle near their base. 



By continued rotation this sharp angle is driven against 

 the membranes forcibly enough to bend them outward in a 

 shallow dent (fig. 6). This keeps the sharp angle in position, 

 and it soon bores its way through the membranes (fig. 7). 



The bunch of flagella is then straightened out, tearing the 

 edges of the rent still more, and some of the liquid which 

 filled the membranes escapes. This causes them to collapse, 

 and they fall against the rotating embryo, whose strong 

 motion quickly tears them in pieces, leaving the larva free. 



This is the second and probably the most critical period in 

 the life of the embryo. Of those reared from artificially 

 fertilised eggs fully one third fail to escape from the mem- 

 branes, and normal embryos fare no better, since 35 per 

 cent, of those under observation perished inside the mem- 

 branes. When we reflect on the apparent ease with which 

 the sperms penetrated these same membranes we are 

 impressed with the delicacy of the apical flagella. Hatschek 

 has noted in the Annelid Eupomatus cruciatus that the 

 cilia pierce the membrane when they first appear, and it 

 would seem as if the latter was not as tough as in Cerebra- 

 tulus (Korscheldt and Heider, p. 173). 



We find examples of both dextral and sinistral rotation, 

 with a large majority in favour of the former. 



The rapidity of rotation and the time at which the larva 

 escapes also vary greatly. If unable to escape at the proper 

 time the continued rotation reacts upon the larva and 

 tears asunder the cells of which it is composed. 



In consequence a ragged bunch of cells may often be seen 

 rotating in the midst of a loose mass of individual cells 

 which have been separated from it, the whole enclosed in the 

 original membranes. Or sometimes the membranes give 



