BIGELOW: EARLY DEVELOPMENT OF LEPAS. 73 



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. auatifera 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 



