BIGELOW: EARLY DEVELOPMENT OF LEPAS. 85 



Cirripedia which have ellipsoidal eggs surrounded by a rigid vitelline 

 membrane. 



The rotation appears to be due to the mechanical relations existing 

 between the dividing ovum and the vitelline membrane. 



The first cleavage is a typical case of unequal cell division ; this is 

 widely at variance with the account given by Groom (see the following 

 review of the literature). 



3. REVIEW OP THE LITERATURE ON THE FIRST CLEAVAGE. 



According to the accounts or figures of Fillippi ('65), Mtinter und 

 Buchholz ('69), Hoek ('76), Lang ('78), Nassonow ('87), and Groom 

 ('94), the first cleavage plane in all the species of Lepadidse and Balan- 

 idse, which have been studied by them, is generally transverse to the 

 chief axis ; but it has been sometimes described as occasionally more or 

 less oblique owing to variation. These investigators noticed that the 

 long axis (chief axis) of the unsegmented ovum coincides with the long 

 axis of the vitelline membrane, and that in the two-cell stage the plane 

 of separation is transverse to that axis. These positions of the egg 

 with reference to the vitelline membrane before and after cleavage led 

 to the view that the first cleavage plane is formed at right angles to the 

 chief axis of the egg, i. e., that cleavage is equatorial. Had the position 

 of the polar cell during and after cleavage been carefully observed, this 

 view would not have gained acceptance. Of the above named authors 

 Groom and Nassonow have figured the polar cell in the two-cell stage, 

 and they represent it as situated in the original position near the 

 rounded end of the vitelline membrane, 90 from the cleavage plane. 



Nussbaum ('87, '90) observed in some ova of Pollicipes cleavage 

 planes in various degrees of obliquity with reference to the vitelline 

 membrane, from nearly longitudinal to transverse. He is the only 

 author who has figured or described a polar cell as lying in the cleavage 

 furrow of the two-cell stage of a cirripede egg. Nussbaum explained 

 these varying positions of the cleavage plane and polar cell with refer- 

 ence to the long axis of the vitelline membrane by assuming that the 

 ovum divides almost longitudinally, and that after division the egg 

 turns within the vitelline membrane. The various positions of the first 

 cleavage plane, which were observed by Nussbaum in different eggs, 

 were assumed to represent phases in the turning of the egg as it rotated 

 from the position in which the forming cleavage plane is nearly longitu- 

 dinal to the final position, in which it is transverse. Nussbaum sug- 



