174 



BIG E LOW. 



[Vol. II. 



ment of several Cirripedia from the standpoint of cytogeny, the 

 writer has recently been able to make observations on the 

 living ova of L. anatifera, which explain the various conflicting 

 observations and determine the relations of the first cleavage 

 plane. Some mechanical effects have also been observed, which 

 seem to contribute something of interest to the study of the 

 mechanics of development. Some notes on the observations 

 are given here ; a detailed account with discussion is reserved 

 for a future paper. 



A brief description of the unsegmented ovum will serve as a 

 basis for the discussion of the cleavasfe. The ovum of L. ana- 



FlG. 



Fig. 2. 



Fig. 3. 



Fig. 4. 



Camera drawings of the living ovum, showing the rotation of plane of cleavage, n-b marks 

 the constant long axis of the vitelline membrane (z'.), / indicates the second polar body, 

 and y the yolk spherules. 



tifera just before the beginning of cleavage is ellipsoidal in 

 form. The yolk spherules at this time lie aggregated at one 

 pole, which is known to be the ultimate posterior ; but during 

 the preparation for division the yolk shifts to one side of the 

 polar area (Fig. i). The vitelline membrane closely surround- 

 ing the ovum is rounded at the anterior and somewhat pointed 

 at the posterior pole. It is apparently quite rigid and does 

 not greatly alter in shape during the embryological develop- 

 ment. The second polar body lies within the vitelline mem- 

 brane at the anterior end of the ovum (Fig. i). 



Figs. 1-4 illustrate the changes in the external form of 

 the ovum during the first cleavage. They are from camera 

 lucida drawings of a living ovum, made at intervals of four 

 minutes. They were selected from a series of fourteen drawings 



