IOOO THE AMERICAN NATURALIST. [Vor. XXXV. 
the uninjured portion. All such cases of partial development 
die in early stages. When the yolk is exposed it appears to ` 
absorb water very rapidly, the egg or the injured part of it 
increases in size and becomes more transparent, and sections 
show vacuoles in place of the yolk gran- 
ules. The death of the egg always 
accompanies or follows this absorption 
of water. Apparently the presence of 
the ectoplasmic layer prevents this absorption of 
water by the yolk. The egg is very sensitive to inju- 
ries involving the rupture of the ectoplasmic layer. 
All attempts to kill a part. of the egg ended in the 
death of the whole a little later. 
The eggs are laid normally in strings of about 
the length of that portion of the body containing 
gonads. The eggs are deposited in two rows, each 
row representing the eggs from one side of the body. 
All the eggs are enclosed as they emerge in slime 
secreted by the dermal glands, and then the worm 
crawls out from between the two rows of eggs which 
remain attached to the substratum. As shown by 
the figure, the eggs at one end of the string are 
placed irregularly, as if the secretion enclosing the 
eggs had been stretched. This end is undoubtedly 
the anterior end of the string, ż.e., the end at which 
the worm leaves after laying, and its form is due 
to the fact that some of the slime adheres to the 
body of the worm, and together with the enclosed 
eggs is dragged after it as it moves away. 
Ess d The cleavage of the eggs is typically spiral, all the 
“mm cells being nearly equal in size during the earlier 
stages. The development is of the direct type. The surface 
of the embryo becomes ciliated, and it moves about within the 
egg membrane for a day or more before leaving it. The spher- 
ical embryo begins to elongate after three or four days, leaving 
the membrane when about one-half millimeter in length. The 
young animal possesses at first four eyes, the posterior pair 
of the adult being formed at a later stage than the others, thus 
