HABITS, ETC., OF CEREBRATULUS LACTEUS. 149 



A segmentation cavity appears very early, but not quite 

 in the eight-cell stage as described by Barrois for Lineus 

 Gesserensis (7). 



As would be readily inferred, the first two blastomeres are 

 easily shaken apart when joined only by the narrow isthmus, 

 and the same is true of the four- and eight-cell stages. 



The experiments of Morgan and others upon frogs' eggs 

 at once suggested that blastomeres separated in this way 

 might develop into larvae. Accordingly experiments were 

 made with the most satisfactory results. Even if the egg- 

 membranes were broken in the shaking and the blastomeres 

 set free, they segmented as though nothing had occurred, 

 and developed into larvae similar in all respects to those 

 reared normally, except that they were only half as large, or 

 even smaller in the case of isolated blastomeres of the four- 

 and eight-cell stages. 



Summary. — 1. The axis of first segmentation is prede- 

 termined in the ovary at a very early stage of develop- 

 ment. It extends from the point where the egg-cell is 

 originally attached to the wall of the ovary, diametrically 

 across the egg to a point where the polar bodies subsequently 

 appear. 



2. The ripe ovum is spherical. Segmentation is total and 

 equal ; the first three cleavage planes occupy the three 

 dimensions of space, the second being horizontal. The 

 first segmentation occupies fifteen minutes, and is followed 

 by a period of rest lasting ten or twelve minutes. The second 

 occupies not more than ten minutes, followed by a rest of 

 five minutes. 



Subsequent divisions proceed regularly but more slowly, and 

 the blastula stage is reached at the end of the fifteenth hour. 



3. The polar bodies remain attached to each other and to 

 the egg through segmentation, and continue their filose 

 activities. 



All evidence indicates that the point at which the second 

 body is attached to the egg remains constant. It appears at 

 the bottom of the first cleavage groove, then changes, as the 



