BIGELOW: EARLY DEVELOPMENT OF LEPAS. 13 
Figures 3 and 18 ; the two nuclei which he describes being evidently the 
pronuclei and not daughter nuclei sprung from the first segmentation 
nucleus. The figures in the present paper show that a segmentation 
nucleus does not exist during the separation of yolk and protoplasm. 
Two pronuclei are in the egg, but they do not appear to fuse completely 
until the nuclear membranes fade away at the beginning of division. 
My figures of the first cleavage show, as opposed to Groom’s description, 
that the nuclei resulting from the first division are not at first both 
located in the upper half of the egg, where the protoplasm is more 
concentrated. 
Nussbaum (’90) observed the two nuclei in Pollicipes as the waves of 
constriction passed over the egg during the separation of yolk and proto- 
plasm, and interpreted them as pronuclei. He figured and described 
the pronuclei as approaching along a line nearly coinciding with the long 
axis of the egg; and he assumed that the plane of the first cleavage is 
perpendicular to the contact surface of the pronuclei. My Figures 18- 
20 confirm his observations on Pollicipes, for it is certain that there are 
two pronuclei in the protoplasmic mass at the animal pole of the egg in 
L. anatifera and L, fascicularis as the separation of yolk and protoplasm 
progresses. I have studied sections of Pollicipes which show similar 
conditions. Nussbaum’s interpretation of these nuclei as pronuclei is 
certainly correct, as is likewise his description of their approach and 
contact. 
V. General Sketch of Cleavage and Germ-Layers. 
The cleavage of Lepas is total, unequal, and regular. Stages of 2, 4, 
8, 16, 32 and 62 cells are normally formed. Cells of a given generation 
may anticipate their sister cells in division, but no second division of 
such cells takes place before all other cells have completed corresponding 
cleavages and reached the same generation. 
The first cleavage plane is nearly parallel to the long axis of the ellip- 
soidal egg, which divides into a small anterior cell (micromere) and a 
large posterior yolk-bearing cell (macromere). The plane of the second 
cleavage is perpendicular to that of the first, a second micromere being 
cut off from the yolk-bearing macromere, while the first micromere divides 
into two of equal size. The plane of the third cleavage is essentially 
perpendicular to both the preceding ones. A third micromere is sepa- 
rated at this cleavage from the yolk-macromere, which is now purely 
mes-entoblastic. Thus by the first, second, and third cleavages three 
