520 PllOCEEDINGS OF THE ACADEMY OF [DeC, 



the animal pole and fa(*ui<i; the cell in question. If the uj^per cell is to 

 the left of the lower the division is Iseotropic ( Lillie, 95). If the spindle 

 is horizontal, i.e., the cleavage meridional, the cell to the right receives 

 the smaller exponent. 



Following Child (1900), the macromeres receive the coefhcient of the 

 quartet to which they last contributed. Thus 3A gave rise at its last 

 division to 3o. Further details of the system will become evident by 

 reference to the tables of cell lineage and to the figures. 



The Livixc; Eca;. 



The living egg of Planocera inquilina consists of a uniformly dense 

 mass of granules which vary only slightly in size, l^etween these 

 granules is a light colored fluid substance. When the one-celled egg 

 is strongly centrifuged for some time the yolk granules are compacted 

 to one side and a cap of the light colored fluid, in which are only a few 

 granules, lies at the opposite side. This cap of fluid occupies perhaps 

 one-fourth or a little less of the entire egg. When the egg is crushed 

 under a cover glass and examined with an immersion lens, minute 

 bodies (microsomes?) are found in the fluid portion. These small 

 bodies exhibit a constant "brownian" movement. When the egg is 

 entire, however, no motion of any kind can be discerned. 



The eggs of this species of Polyclad appear perfectly uniform through- 

 out. Helenka, Goette and Lang have found that in many polyclad 

 eggs there is a darker iiuicr portion and a lighter outer layer to the 

 eggs. I could make out no such differentiation in these eggs. Lang- 

 did not find this separation of substance in Discoadis tigrina. 



Consideral)le time was spent during the summer attempting to 

 experiment on these eggs. But in all cases where the conditions were 

 varied from the normal the egg dieil in a short time and no results were 

 obtained. 



All the early cleavages as well as the matiu'ation divisions occur at 

 intervals of about one hour. The whole development proceeds rather 

 more rapidly than in most })olyclads. At the end of the second day 

 or at the beginning of the third the embryo is completely covered by 

 the small ectodermal cells. Cilia soon begin to form on these and by 

 the third day the embryo begins to slowly I'otate within the capsule. 

 During the next day or two the cilia become better developed and the 

 embryo rotates faster and faster. The rotation takes place first in one 

 direction and then after a short pause in another. Occasionally they 

 cease moving for S(niie time. The eye spots appear al:)Out the fourth 

 (lay. During the foui'th and fifth days a number of homogeneous 



