BIGELOW: EARLY DEVELOPMENT OF LEPAS. 69 



and finally .results in a telolecithal arrangement of the materials of the 



egg- 



Eggs taken from the egg-lamellse at all phases of the maturation have 

 been carefully compared with the corresponding stages of isolated eggs 

 which were kept in watch glasses. The distortions in form produced by 

 pressure apparently do not disturb the normal course of cytological 

 changes in the egg. 



Figures 1-6 represent a series of camera sketches made from a living 

 egg at intervals within a period of three hours. In Figure 1 the egg is 

 represented just at the completion of the separation of the second polar 

 cell. The egg is approximately spherical and closely surrounded by the 

 vitelline membrane (mb.vt.). The yolk with its oil globules is in general 

 uniformly distributed, but already some of the globules have been seen 

 to move towards the vegetative pole. Figure 2 shows the well-marked 

 beginning of elongation ; the yolk is collecting at the vegetative pole and 

 a mass of protoplasm, concentrating into the animal half of the egg, is 

 dark and granular. Figure 3 represents a stage some minutes later. A 

 circular depression has appeared around the egg at the equator constrict- 

 ing the egg into nearly equal lobes. The upper, protoplasmic lobe is dark 

 and granular, especially near its centre, whereas the lower or yolk-lobe is 

 relatively clear and transparent, as represented in Figure 18 (Plate 2). 

 The constriction now moves toward the vegetative pole of the egg, where 

 the yolk is collecting (Fig. 4). Gradually the constricting furrow dis- 

 appears (Fig. 5), and the egg becomes ellipsoidal, as shown in Figure 6. 

 At the animal pole the egg continues to be bluntly rounded, while at the 

 vegetative pole it becomes more pointed. The vitelline membrane, hav- 

 ing taken on this shape, retains it throughout the development, and 

 appears to be quite rigid from this stage onward. At the close of the 

 elongation the upper, animal portion of the egg is largely composed 

 of dark granular protoplasm containing some small granules of yolk, 

 but no oil globules (Plate 2, Figs. 19, 20). The lower vegetative 

 part of the egg is more transparent and contains the mass of yolk gran- 

 ules. The oil globules are concentrated at the pointed end of the egg 

 and for a time are arranged in strict radial symmetry with respect to the 

 long (chief) axis of the egg. Protoplasmic strands extend throughout 

 the vegetative half of the egg. 



The elongation of the egg and the separation of yolk and protoplasm, 

 which result in the telolecithal condition and the establishment of visible 

 polarity, are entirely distinct from the first cleavage processes, with which 

 Groom ('94) has confused them (see review of the literature on first 



