BIGELOW: EARLY DEVELOPMENT OF LEPAS. 77 



VII. Cleavage. 



1. Introductory. 



The following description of the cleavage of the egg of Lepas applies 

 particularly to L. anatifera, of which I obtained abundant material of all 

 stages in 1898, being thus able to study the early development in con- 

 siderable detail. An extensive series of the eggs of L. fascicularis was 

 later obtained and its development has been carefully compared with 

 that of L. anatifera. There is such close parallelism in the development 

 of the two species that the following account will apply in all important 

 respects to L. fascicularis as well as to L. anatifera. Figures 95—126 

 (Plates 11, 12) of L. fascicularis when compared with those of L. anati- 

 fera show how close is the similarity between the two species. At the 

 close of this chapter (p. 117) there are some notes on the early develop- 

 ment of L. fascicularis which supplement and correct a preliminary 

 account of this species published by me in 1896. 



The principal stages in the development of L. pectinata and L. hillii 

 have also been examined, but their development does not appear to differ 

 in any important respects from that of L. anatifera and L. fascicularis. 



2. First Cleavage. Two Cells. 



The first cleavage of the egg of all Lepadidse and Balanidse whose 

 development has been heretofore described results in the formation of 

 two unlike cells. The smaller cell, rich in protoplasm, is situated at the 

 rounded end of the vitelline membrane ; the other, laden with yolk, at 

 its pointed end (Plate 1, Fig. 16). In previous accounts the first cleav- 

 age plane has usually been described as being formed perpendicularly 

 to the long axis (chief axis) of the egg. The first cleavage plane has, 

 accordingly, been characterized as equatorial, and the long axis of the 

 two-cell stage has been regarded as identical with the long axis (chief 

 axis) of the unsegmented egg. 



In the following account ^ it will be shown that the first cleavage fur- 

 row appears approximately in the long axis (chief axis) of the egg ; and 

 that, therefore, the first cleavage is meridional, not equatorial as was 

 hitherto supposed. It will be shown, further, that the position of the 

 cleavage plane in the two-cell stage is due to a rotation of the dividing 



^ Some notes on the first cleavage of L. anatifera have already been published 

 (Bigelow, '99). 



