CLE A VAGE IN ANNELIDS. 



95 



I shall make a somewhat detailed comparison of Amphitrite} 

 which has been studied by Dr. Mead, as an example of unequal, 

 and Podarke, on which I have been working for some time, as 

 an example of equal cleavage. In the unequal type, to which 

 most of the annelids and all of the gasteropods and lamelli- 

 branchs which have been studied belong, frequently at the two-, 

 and always at the eight-cell stage, differences in size appear 

 among the segments ; and, supposing the first and second cleav- 

 age planes to divide the body into four parts, the quadrants are 

 never exactly symmetrical ; so that, from the very beginning, 

 orientation is comparatively a simple matter. In Amphitrite 



Fig. I. — Amphitrite, four-cell stage. 



Fig. 2. — Podarke, four-cell stage. 



the first division is unequal, and this difference in size is con- 

 tinued into the four-cell stage (Fig. i).^ Here one cell is 

 very much larger than either of the other three, and lies at 

 what will be the posterior end of the future embryo. Notice 

 here that the second plane of cleavage is not a straight line, 

 but it cuts the first in two points some distance apart. The 

 portion of the second furrow between these two points is the 

 so-called " cross furrow," or " Brechungslinie." I shall follow 

 Conklin in calling it the " polar furrow." It is obviously 

 formed by the rotation of cells A and C to the left, so that 

 they lie partly over B and D. In Amphitrite, as in other 



1 Mead, " Development of Marine Annelids," /<?2/r«. of Morph., vol. xiii. 



2 I have modified slightly the lettering in the figures taken from Mead to corre- 

 spond with a nomenclature proposed for Podarke. 



