120 ELEVENTH REPORT. 



THE GERM-CELL DETERMINANTS OF CHRYSOMELID BEETLES. 



Part L The Germ-Cell Determinants. ' 



Part II. The Results of Removing the Germ-Cell Determinants. - 

 ^ . , ^ By R. W. Hegner. 



j; r Part I. The Germ-Cell Dp:terminants. 



This paper is based on the study of the embryology of four chrysomelid 

 beetles. Most of the work was done on the eggs of Calligrapha multipvnctata, 

 a beetle that lives on the leaves of the long-leaved willow; similar conditions 

 were also found in the eggs of two other members of this genus and in the 

 potato beetle, Leptinotarsa decemlineata. 



At the time of laying (Fig. 1), the eggs of these beetles consist of a large 

 central mass of yolk globules (y) and a thin peripheral layer of cytoplasm 

 (khbl) which is connected with the cytoplasm filling the interdeutoplasmic 

 spaces. Polar body formation is taking place at this time, and there are 

 one or more sperm nuclei near the anterior end of the egg. The peripheral 

 layer of cytoplasm is homogeneous except at the posterior end of the egg. 

 A disc shaped mass of granules (pd. g) is present at this point suspended 

 in the outer cytoplasmic layer. I have called this mass of granules the 

 pole-disc and the individual granules, the germ-cell determinants. The 

 pole-disc covers about one-eighth the area of the posterior end of the egg. 

 The granules stain deeply in haematoxylin. 



The clevage products in the eggs of these beetles migrate from within the 

 central yoke mass, where the first divisions take place, to the periphery. 

 Here they fuse with the cytoplasm ; cell walls are formed and a blastoderm 

 is produced of a single layer of cells. Conditions are not the same at the 

 ])osterior end of the egg. At this point (Fig. 2) a certain number of the 

 cleavage products come in contact with the germ-cell determinants. These 

 nuclei do not remain at the periphery to become the centers of blastoderm 

 cells; but, gathering a covering of granules about them, they continue their 

 migration until they have become entirely separated from the egg. These 

 cells, the primordial germ-cells (pc), now lie l^etween the egg and the vitelline 

 membrane. A superficial view of the posterior end of an egg at this time 

 reveals the fact that there are 16 germ-cells present and that these 16 cells 

 have carried practically all of the germ-cell determinants out of the egg. 

 These 16 germ-cells by two rapid divisions increase to 64. During the divi- 

 sions the granules are ec[ually distributed among the daughter cells. No 

 further divisions take place until a late stage in embryonic development. 

 The germ-band arises as follows. The blastoderm on the ventral surface 

 of the egg thickens to form the ventral plate; a longitudinal concavity, the 

 ventral groove, appears in the center of this plate. The ventral groove 

 becomes deeper at the posterior end of the egg and the germ-cells are drawn 

 into this concavity by the contraction of the ventral plate. The ventral 



lA full account of the orig:ii and early historj' of the gemi-cells' in chrysomelid l)eetlf's is now in 

 press (Journal of Moiphology), 



2A preliminary report on this siihje:'t appeared in the Biological Bulletin, Vol. 16 , Xo. 1, De- 

 cember, 1908. 



