652 HAROLD HEATH, 



'93. Metcalf, M. M., Contribution to the embryology of Chiton, in: 



Stud. biol. Lab. Johns Hopkins Univ. 

 '93. PiLSBRY, H. A., Manual of conchology, V. 14 and 15. 

 '86. Patten, W., The embryology of Patella, in: Arb. zool. Inst. Wien, 



V. 6. 

 '95. Roux, W., Gesammelte Abhandlungen über Entwicklungsmechanik 



der Organismen. 

 '93. Stauffachee, H., Eibildung und Furchung bei Cyclas cornea, in: 



Jena. Z. Naturw., V. 28. 

 '97. Trbadwell, A. L., The cell-lineage of Podarke obscura, in: Zool. 



Bull., V. 1, No. 4. 

 '90. Watase, S., Studies on Cephalopods. I. Cleavage of the ovum, 



in: J. Morph., V. 4. 

 '87. Whitman, C. 0., The embryology of Clepsine, in : J. Morph., V. 1. 

 '89. W^iLSON, E. B., The embryology of the earthworm, in: J. Morph., 



V. 3. 

 '92. — The cell-lineage of Nereis, ibid. V. 6. 

 '91. V. WiSTiNGHAUSEN, C, Untersuchungen über die Entwicklung von 



Nereis Dumerilli, in: Mitth. zool. Stat. Neapel, V. 10. 



Description of Figures. 



Plates 31—35. 



All the figures are drawn with a camera lucida on the same scale 

 with the exception of the last eight figui'es in Plate 35. The position 

 of the nuclei and cell walls have been drawn as accurately as possible. 

 The letters A, B represent the left and right anterior while S and C 

 indicate the right and left posterior quadrants. 



1 a, lb, 1 c, Id, 1 a^ etc. first quartette of ectomeres 



2a, 2h, 2c, 2d, 2a^ etc. second „ „ „ 



3a, 3h, 3c, 3d, Sa^ etc. third „ „ „ 



4a, 4 1), 4c, 4d, 4a ^ etc. fourth „ „ „ 



M.3Î mesoblast pole cells. 



In the cleavage of a cell the upper product receives the smaller ex- 

 ponent thus: 



i 1 //^«^-^ 



