BIGELOW : EARLY DEVELOPMENT OF LEPAS. 79 



In those eggs in which it is obUque at the close of the first cleavage, 

 the vitelline membrane appears relatively broader, and the divided 

 ovum is easily adjusted to an oblique position within the membrane. 



Fifteen or twenty minutes usually elapse between the first external 

 appearances of division and the complete separation of the cells. From 

 the cases which I followed continuously it appears that the cleavage 

 beo-ins within two to three hours after the formation of the second polar 

 cell. 



During this cleavage the ova are seen to undergo a series of marked 

 contractions, as shown in Figures 11 and 14. Immediately following 

 each contraction the cleavage furrow deepens and the ovum rotates 

 tiirough several degrees. These phenomena are probably due to the 

 action of the astral fibres, which, as will be shown later, are a well- 

 marked feature of the cleaving ovum. The external appearances would 

 lead one to think that the internal contractions occur spasmodically 

 rather than continuously. Similar appearances were many times noted 

 also in the later cleavages. 



Additional evidence in support of this observation concerning rotation 

 of the dividing egg has been obtained from living eggs of L. fascicularis 

 and a species of Balanus. In L. fascicularis (Plate 11, Figs. 95-97) the 

 first polar cell has been observed to remain attached to the vitelline 

 membrane at its blunter pole until after the close of the first cleavage, 

 when the second polar cell, attached to the egg, has moved 90'' from the 

 blunt pole of the vitelline membrane. This observation is conclusive 

 confirmation of my earlier observations on L. anatifera. 



While no observations have as yet been made on the living ova of 

 species of Cirripedia other than those already mentioned, the study of 

 preserved material of other species indicates that in these the first cleav- 

 age takes place as in L. anatifera and in L. fascicularis. In L. hillii, L. 

 pectinata, Pollicipes, and Balanus the chief axis coincides with the long 

 axis of the unsegmented ovum and of the vitelline membrane. After the 

 first cleavage, I find the polar cell in the cleavage furrow, which approx- 

 imately coincides with a transverse plane of the vitelline membrane. 



So far as known similar relations exist between the ovum and the 

 vitelline membrane before and after cleavage in the ova of all Eucirri- 

 pedia ; therefore, it is very probable that cleavage takes place in the 

 entire group as in L. anatifera. Van Beneden's ('70) figures of Saccu- 

 lina suggest that the same may also be true for the ova of Rhizocephalan 

 Cirripedia. 



The internal phenomena connected with the cleavage could not be 



