NOTES ON THE FIRST CLEAVAGE OF LEPAS. 



MAURICE A. BIGELOW. 



All previous observers of the development of the Cirripede 

 ova have agreed that the polar bodies are formed at the proto- 

 plasmic end of the ellipsoidal ovum. The first cleavage plane 

 has usually been described as almost transverse to the long 

 axis of the ovum. Ova in which the first cleavage plane was 

 oblique and sometimes almost parallel to the long axis have 

 been described as occasionally occurring. These have been 

 regarded as variations from the normal cleavage. The conclu- 

 sion has seemed unavoidable that the first cleavage plane is 

 equatorial, and that it does not pass through the animal pole, 

 as is the case in nearly all ova. Such are the conclusions of 

 Groom ('94),i the latest investigator of the development of 

 Cirripedia. The writer ('96)2 found that in Lepas fascicidaris 

 the second polar body lies in the first cleavage furrow, the first 

 one being outside the vitelline membrane. This position of 

 the second polar body indicated that the first cleavage furrow 

 passed through the animal pole of the ovum, but no observa- 

 tions were made which explained this position of the polar 

 body apparently 90° removed from the point of its formation. 

 Nussbaum ('87)3 found similar relations existing in the ovum 

 of Pollicipes, and assumed that the first cleavage furrow forms 

 in the long axis of the ovum passing through the animal pole, 

 and that it later rotates to the transverse position. The evi- 

 dence which he offered in favor of his assumption was appar- 

 ently shown to be incorrect by the subsequent observations 



of Groom. 



In the course of comparative studies of the early develop- 



1 Groom, T. T. ('94), " Early Development of Cirripedia," Phil. Trans. Vol. 

 clxxxv B, pp. 1 19-232. 



2 Bigelow, M. A. ('96), " Early Development of Lepas fascicularis," a pre- 

 liminary note, Anat. Attz. Vol. xii, pp. 263-269. 



3 Nussbaum, M. ('87), " Vorlaufiger Bencht " Sitz. Berli7i Akad. 1887. pp. 

 1051-55. — ('90), " Anatomische Studien an Californischen Cirripedien." Bonn. 



