No. 3-] APPENDIX. 459 



fcetida) the first two cleavage-planes are oblique to the plane of 

 symmetry, as in Clepsine or Rhynchelmis, and differ from their 

 position in Nereis. This difference is, however, caused simply 

 by late displacements of the entomeres, and does not affect the 

 earlier stages ; it is, therefore, without morphological signifi- 

 cance. 



(8) The blastopore assumes a triangular form by the narrow- 

 ing of its posterior portion. It appears to close completely from 

 behind forwards, and the mouth afterwards appears at the point 

 of final closure, i.e. in front. 



(9) In Aricia fcetida the primary mesoblasts pass into the 

 interior as the blastopore closes. 



I have followed up to the thirty-two-celled stage the cleavage 

 of a series of Hydroides ^ which agrees closely in development 

 with the "Eupomatus uncinatus " studied by Hatschek and is 

 of special interest as a representative of the so-called "equal 

 cleavage " among annelids. The order and direction-planes of 

 the divisions are nearly the same as in the other forms, but the 

 spiral symmetry is complete up to the latest stage observed. 

 The first four blastomeres, A, B, C, D, are exactly equal. At 

 the third cleavage four slightly smaller upper cells, a^, b^, c^, d'^ 

 ("micromeres ") are separated, in a right-handed spiral, from 

 four lower "macromeres." The fourth cleavage agrees closely 

 with that of Polymnia, Nereis, etc., but the micromeres of the 

 second group, c^, b^, c^, d"^, are of exactly equal size, and are but 

 little smaller than the macromeres. The fifth cleavage is like- 

 wise essentially like that of the other forms, and the third set 

 of ectomeres, a^, b^, <?, d"^, are much smaller than the second set 

 {a^, b^, etc.). 



The embryo now (thirty-two-celled stage) agrees essentially 

 in the arrangement of the blastomeres with the corresponding 

 stage of the "unequal type" {Nereis, etc.), but differs from it 

 (a) in the equal size of the four macromeres, and {b) in the lack 

 of a differentiated first somatoblast {i.e. d^ is no larger than 

 d^, b"^, c^). It is at this period that the second somatoblast, M, 

 is formed in the unequal type, where, as I have shown above 

 {Polymnia, Aricia fcetida), it forms one of the four cells {a^, b^, 



1 The species is related to Hydroides (" Enpomatus ") tmcinatus, but differs from 

 it both in the looser and more delicate growth of the tube, and in the more numerous 

 divisions in the crown of the operculum. 



